To determine the only correct sentence among the given options, let's analyze each statement:

A. All of the sentences below are correct.
- If this were true, then statement B, C, D, and E would also have to be correct, which would create a contradiction.

B. All the sentences below are wrong.
- If this were true, then statement C, D, and E would all be wrong. However, if statement E is wrong, that means at least one above it (likely B) is correct, creating a contradiction.

C. Only one sentence above is correct.
- If this is true, then this statement could be the only correct one.

D. All the sentences above are correct.
- If this were true, then A, B, and C would also be correct, which leads to contradiction.

E. None of the sentences above is correct.
- If this were true, then by negating A, B, C, and D, statement C can still possibly be correct, hence causing a contradiction.

From the analysis, the only statement that stands without contradiction is C.

Therefore, the letter before the correct statement is **C**.