If the sum of binomial coefficients in the expansion of (x + y)^n...

The sum of binomial coefficients in the expansion of $(x + y)^n$ is $2^n$.

$2^n = 256$

$2^n = 2^8 \Rightarrow n = 8$

In the expansion of $(x + y)^8$, the number of terms is $n + 1 = 9$.

The greatest binomial coefficient occurs at the middle term.

Since $n = 8$ is even, the middle term is the $(\dfrac{n}{2} + 1)$-th term.

Middle term $= \dfrac{8}{2} + 1 = 5$

The greatest binomial coefficient occurs in the fifth term.

Answer: Fifth