How can I see the diff of two commits with respect to another one?

Example: There is a commit A and two branches from A: B and C. In the end, B and C are merged into D. I want to see if the difference between D and B is exactly the changes made in C.

      -- B --
    /        \
A ------ C --- D

The problem I have is that the merge D has a bug related to the changes made in B which were definitely not there when B was developed.

Solution

I want to see if the difference between D and B is exactly the changes made in C

Sounds like you want git range-diff

git range-diff B..D A..C