a) If today is Monday, then tomorrow is Tuesday.
b) If today is not Monday, then tomorrow is not Tuesday.
c) If tomorrow is not Tuesday, then today is not Monday. 

We go only by what we can tell from the statements only,
not using our knowledge of the days of the week.

Let p = "Today is Monday" and q = "tomorrow is Tuesday"

a) If p, then q.
b) If not p, not q.
c) If not q, then not p. 

The answer is c) is the equivalent contrapositive of a)

b) is not equivalent because it is the inverse of a).

Edwin