Concept:

• Radioactivity: Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. A material with an unstable nucleus is considered radioactive.
• A radioactive nucleus has an unstable group of protons and neutrons that becomes more stable by emitting an alpha particle, a beta particle, or a gamma photon.
• Atoms are radioactive if their nuclei are unstable and spontaneously (and randomly) emit various particles α, β, and/or γ radiations.
• The basic unit for measuring the amount of radioactivity was the curie (Ci) - which was originally defined as equivalent to one gram of radium-226.
• 1 curie = 3.7x10¹⁰ radioactive decays per second.
• In the International System of Units (SI), the curie has been replaced by becquerel (Bq).
• 1 becquerel = 1 radioactive decay per second = 2.703x10^-11Ci

Interpretation:

• From the above discussion, the SI unit of radioactivity is becquerel (Bq).
• The correct answer is option 4.