The correct answer is (A).
The (R)-enantiomer of an amino acid is the one that has the amino group (-NH2) on the right side of the carbon atom that is attached to the carboxyl group (-COOH) and the R-group. In (A), the amino group is on the right side of the carbon atom that is attached to the carboxyl group and the R-group is a methyl group (-CH3). In (B), the amino group is on the left side of the carbon atom that is attached to the carboxyl group and the R-group is a hydrogen atom (-H). In (C), the amino group is on the right side of the carbon atom that is attached to the carboxyl group and the R-group is a carboxylic acid group (-COOH). In (D), the amino group is on the left side of the carbon atom that is attached to the carboxyl group and the R-group is an ethyl group (-CH2CH3).
Here is a diagram of the (R)-enantiomer of alanine:
The (R)-enantiomer of alanine is the most common enantiomer of alanine found in nature. It is also the enantiomer that is used in most biochemical studies.