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Homotopic means identical. If replacing two protons with a different group (X) gives the same compound then protons are homotopic.

If by replacing two protons with two different groups if the pair shows an enantiomeric relationship then protons are enantiotopic.

If replacing two protons with different groups gives a pair of diastereomers then the protons are diastereotopic.

Homotopic and enantiotopic protons gives one signal.If the asymmetric center is present then the protons of${\text{-CH}}_{\text{2}}$are diastereotopic.Diastereotopic protons give different signals.

The above structure contains a chiral center. So the hydrogens of ${\text{-CH}}_{\text{2}}$ group are diastereotopic and give different signals. While ${\text{-CH}}_3$protons are homotopic and give one signal.

Signals given by S-malate is 6