Which one of the following is a factor of the sum of first twenty-five natural numbers ?

Answer: Option C

Sum first n natural numbers
$$\text{ = }\left[\frac{\text{n(n + 1)}}{2}\right]=25\times 13$$
Sum of first 25 natural numbers
$$\text{ = }\frac{25(26)}{2}=25\times 13$$
Now check option
Clearly, we can see 13 is factor
So, '13' is answer