Question 950421
The first step I'd always to prove that the formula/equation is true for n=1, so there should be no n in your first step.
That disqualifies choices b, c, and d, so it must be a.
Also, substituting 1 for n in that formula tells you that the sum of the first 1 natural numbers,
from 1 to n=1, which is just 1,
is equal to 1(1+1)/2.