Units check: which of the following is a unit of radioactivity (activity of a radioactive source)?

Introduction / Context:
Physical units in nuclear engineering and physics are often confused because many are used in adjacent subfields. Radioactivity (activity) measures the rate of nuclear disintegrations per unit time. Recognising the correct unit avoids conceptual errors and helps interpret measurements and safety limits.

Given Data / Assumptions:

• Activity is the number of decays per second.
• Two common units are used historically: the Curie (Ci) and the SI unit Becquerel (Bq).
• Other units in the options belong to different quantities or scales.

Concept / Approach:
The Curie (Ci) is defined historically as 3.7 × 10^10 disintegrations per second, approximately the activity of 1 gram of radium-226. The SI unit of activity is the Becquerel (1 Bq = 1 disintegration per second), but since Becquerel is not an option here, Curie is the correct choice. Barn is an area unit used for nuclear cross-sections, Fermi is a length scale for nuclear dimensions, Angstrom is an atomic length unit, and Tesla is magnetic flux density in electromagnetism.

Step-by-Step Solution:
Identify what radioactivity quantifies: disintegrations per second.Recall the historical and SI units: Curie and Becquerel.Select Curie because Bq is absent from options.

Verification / Alternative check:
Any standard table of nuclear units lists Curie and Becquerel as activity units, confirming Curie as valid when Bq is not provided.

Why Other Options Are Wrong:
Barn: unit of nuclear cross-section (area).Fermi: unit of length (~10^-15 m).Angstrom: unit of length (~10^-10 m).Tesla: magnetic field strength unit, not radioactivity.

Common Pitfalls:
Assuming “barn” relates to activity because it is used in nuclear contexts; remember it measures effective target area for interactions, not decay rate.

Final Answer:
Curie