Answer: B — 5
Since $5^1=5,\ 5^2=25,\ 5^3=125,\ldots$, every power of 5 ends with the digit 5 for any natural number $n$.
The units digit of powers of 5 follows a fixed pattern: it is always 5. This is because $5\times5$ always gives a product ending in 5. The prime factorisation of $5^n$ contains only the prime 5, so it can never end in 0 (which would require factors of both 2 and 5). Source: Chapter 1, Section 1.2 (Fundamental Theorem of Arithmetic).