Answer :
A and D are enantiotopic and B and C are homotopic.
If replacing two protons with a different group (X) gives the same compound, the protons are called Homotopic. If replacing two protons with a different group (X) gives a pair of enantiomers, the protons are called Enantiotopic.
Homotopic, simply means identical. For example, all the protons in ethane are homotopic. Even tough each proton is physically different, but we say that they are identical because the rotation about the C-C single bond is so fast that averagely speaking all the protons appear in the same environment.
Replacing the hydrogens in order to determine the relationship of the protons and hence the number of NMR signals may look overwhelming and confusing and if it all seems too much for determining the relationship of the protons and the number of expected signals.
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