We are given that N1,000 accumulates to N1,240 in 3 years at a certain simple interest rate percent per annum, and we want to find this interest rate.

The formula for calculating the final amount A after t years at a simple interest rate r percent per annum on a principal amount P is:

A = P + Prt/100

In this case, we have A = N1,240, P = N1,000, and t = 3. Substituting these values into the formula above and solving for r, we get:

N1,240 = N1,000 + N1,000 r 3/100

N240 = N30r

r = 240/30

r = 8

So the simple interest rate percent per annum at which N1,000 accumulates to N1,240 in 3 years is 8%, which corresponds to answer choice B. 8%.