The major stereoisomer obtained in the reaction of (S)-2 phenylpropanal with MeMgBr is (S)-2-methyl-1-phenylpropan-1-ol. This is because the reaction proceeds via an SN2 mechanism, which is stereospecific. In an SN2 reaction, the nucleophile attacks the carbon atom from the backside, and the leaving group departs from the same side. This results in inversion of stereochemistry at the carbon atom.
In the case of (S)-2 phenylpropanal, the nucleophile is MeMgBr, and the leaving group is bromide ion. The attack of MeMgBr from the backside results in the formation of an intermediate with a tetrahedral carbon atom. The bromide ion then departs from the same side, resulting in the formation of the product (S)-2-methyl-1-phenylpropan-1-ol.
The other options are incorrect because they do not have the correct stereochemistry. Option A is (R)-2-methyl-1-phenylpropan-1-ol, which has the opposite stereochemistry as the product. Option B is (S)-2-methyl-2-phenylpropan-1-ol, which has the wrong position of the methyl group. Option C is (R)-2-methyl-2-phenylpropan-1-ol, which has the wrong position of the methyl group and the wrong stereochemistry.