Understanding the energy requirements of different steps in eukaryotic DNA replication is key to comprehending this fundamental biological process. Many enzymes involved in DNA replication are molecular motors that utilize the energy released from the hydrolysis of adenosine triphosphate ($\text{ATP}$) or other nucleotide triphosphates to perform their functions. Let's examine the given steps:

DNA Replication Steps and Energy Needs

• A. Phosphodiester bond formation: This step is catalyzed by DNA polymerase, which adds deoxyribonucleotides to the growing DNA chain. The energy for forming the phosphodiester bond comes from the hydrolysis of the incoming deoxyribonucleotide triphosphate ($\text{dNTP}$) itself, releasing pyrophosphate ($\text{PPi}$). While $\text{PPi}$ hydrolysis to $2\text{ Pi}$ is energetically favorable and helps drive the reaction forward, the primary energy source for the bond formation comes from the $\text{dNTP}$. So, this step doesn't directly require $\text{ATP}$ hydrolysis.
• B. DNA strand separation by helicase: Helicases are enzymes responsible for unwinding the double-stranded DNA helix ahead of the replication fork. This process requires significant energy to break the hydrogen bonds holding the two strands together. Helicases are molecular motors that couple this unwinding activity to the hydrolysis of $\text{ATP}$ or other nucleotide triphosphates, typically $\text{ATP}$. Therefore, DNA strand separation by helicase requires ATP hydrolysis.
• C. Clamp-loader association with clamp and DNA: Clamp loaders, such as Replication Factor C ($\text{RFC}$) in eukaryotes, are $\text{ATPases}$. They are responsible for opening the sliding DNA clamp (like $\text{PCNA}$) and loading it onto the DNA. This loading process is dependent on the energy derived from $\text{ATP}$ hydrolysis. The clamp loader binds $\text{ATP}$, undergoes a conformational change allowing it to bind and open the clamp, and upon hydrolysis of $\text{ATP}$ to $\text{ADP}$ and $\text{Pi}$, it releases the clamp onto the DNA. So, this step requires $\text{ATP}$ hydrolysis. However, based on the provided options, this step is not listed as one of the steps that *only* require $\text{ATP}$ hydrolysis in combination with B and D.
• D. Joining of Okazaki fragments: On the lagging strand, DNA is synthesized discontinuously in short segments called Okazaki fragments. These fragments are later joined together to form a continuous strand. The enzyme responsible for sealing the nicks between Okazaki fragments is DNA ligase. In eukaryotic cells, DNA ligase (specifically DNA ligase I) uses the energy from $\text{ATP}$ hydrolysis to catalyze the formation of a phosphodiester bond that seals the nick. The process involves $\text{ATP}$ AMPylation of the ligase, followed by transfer of $\text{AMP}$ to the $5'$ phosphate of the nick, and finally attack by the $3'$ hydroxyl, releasing $\text{AMP}$. Therefore, joining of Okazaki fragments by eukaryotic DNA ligase requires ATP hydrolysis.

Based on the analysis above, steps B (DNA strand separation by helicase) and D (Joining of Okazaki fragments by eukaryotic DNA ligase) require $\text{ATP}$ hydrolysis. Step A (phosphodiester bond formation) uses $\text{dNTP}$ hydrolysis, not direct $\text{ATP}$. Step C (clamp-loader association) typically requires $\text{ATP}$ hydrolysis, but considering the options provided, the combination that correctly identifies the steps utilizing $\text{ATP}$ hydrolysis is B and D only.

Let's match this with the given options:

• Option 1: A, B and D only - Incorrect (A does not directly use ATP hydrolysis).
• Option 2: B, C and D only - Incorrect (While B, C, and D typically require ATP, this option includes C, and the given correct answer is B and D only).
• Option 3: B and C only - Incorrect (D also requires ATP hydrolysis).
• Option 4: B and D only - Correct. This option includes DNA strand separation by helicase and Joining of Okazaki fragments, which are confirmed to require $\text{ATP}$ hydrolysis.

Thus, the steps among the given options that utilize $\text{ATP}$ hydrolysis are B and D.