Answer:
A- conjugate acid
B- conjugate base
C- conjugate base
D- conjugate acid
Step-by-step explanation:
According to the Brownstead-Lowry definition of acids, an acid donates a proton while a base accepts the proton.
If we look at the species ABCD, species A and D donate protons while species B and C accept protons.
C is the conjugate base of A while D is the conjugate acid of B, hence the answer.