Find the odd number / letters / word from the given alternative:

The answer is C. NOL.

The other three options, DEB, HIF, and RTP, all have the property that the sum of the first and third letters is equal to the second letter. For example, in DEB, the sum of D and B is E. In HIF, the sum of H and F is I. In RTP, the sum of R and T is P. However, in NOL, the sum of N and L is not equal to O.