A

The inverse square law states that the illuminance (E) at a distance (d) from a point light source is inversely proportional to the square of that distance. Mathematically, this is expressed as:
$$ E = \frac{I}{d^2} $$
where I is the luminous intensity of the source.
Step 1: For the inverse square law to apply, the light source must emit light uniformly in all directions.
Step 2: An isotropic point source is defined as one that radiates light equally in all directions, fulfilling the conditions of the inverse square law.
Step 3: In contrast, cylindrical sources and search lights do not emit light uniformly in all directions and thus do not follow the inverse square law.
Therefore, the correct answer is A.