It is A. Number of neutrons can differ with and be the same any given element.
Explanation:
In the parallel combination, the equivalent resistance is given by :

4. When three 150 ohms resistors are connected in parallel, the equivalent is given by :

5. Three resistors of 20 ohms, 40 ohms and 100 ohms are connected in parallel, So,

Hence, this is the required solution.
D. distance
A light-year is the distance light would travel in 1 year.