The question asks us to identify the correct active voice form of the given passive voice sentence.

Understanding Passive vs. Active Voice

Sentences can be written in either the passive voice or the active voice. The voice of a verb shows whether the subject performs the action (active) or receives the action (passive).

• Passive Voice: The subject receives the action. The structure often involves a form of the verb 'to be' plus the past participle, and sometimes includes the performer of the action using 'by'.
• Active Voice: The subject performs the action. The structure is typically Subject + Verb + Object.

Analyzing the Original Sentence

The original sentence is: "This error could have been avoided by careful planning."

• Subject: This error (receives the action)
• Verb Phrase: could have been avoided (passive structure: modal + have + been + past participle)
• Agent (Performer): careful planning (introduced by 'by')

This sentence is in the passive voice because "This error" is the recipient of the action "avoided". The performer of the action is "careful planning".

Converting to Active Voice

To convert a passive sentence to active voice, we follow these steps:

1. Identify the agent (the performer of the action, usually after 'by'). This will become the new subject.
2. Identify the original subject. This will become the object of the new sentence.
3. Change the verb phrase to its active form. Remove 'be' verbs (like 'been') and adjust the main verb.

Applying these steps to our sentence:

• Agent becomes Subject: Careful planning
• Original Subject becomes Object: this error
• Verb Phrase Conversion: "could have been avoided" (passive) becomes "could have avoided" (active). The modal verb "could" remains, followed by "have" and the past participle "avoided".

Putting it together, the active voice sentence is: "Careful planning could have avoided this error."

Evaluating the Options

Let's examine each option:

1. $
   This error could be avoided by careful planning.
   $ - This is still in the passive voice and uses a different tense ("could be avoided" vs. "could have been avoided").
2. $
   Someone could have avoided this error.
   $ - This is active voice, but the agent is changed to the generic "Someone", losing the specific information about "careful planning".
3. $
   This error could not have been made.
   $ - This is passive voice, changes the verb from "avoided" to "made", and adds negation ("not"), altering the original meaning.
4. $
   Careful planning could have avoided this error.
   $ - This correctly identifies "careful planning" as the subject (the agent from the passive sentence) and uses the active verb form "could have avoided", with "this error" as the object. This matches our derived active voice sentence.

Conclusion

Option 4 correctly transforms the passive voice sentence into its active form by making the agent ("careful planning") the subject and adjusting the verb phrase accordingly.