Answer: Choice (A) is correct.
The term caveat emptor means ‘let the buyer beware’. The relationship between this pair is whom the phrase is referring to. In choice (B), corpus delecti means the ‘body of a crime’, which does not have anything to do with an army. In choice (C), ex parte means ‘from a partisan point of view’, and you would not expect a juror to be partisan, or biased. In choice (D), in loco parentis means ‘in the place of a parent’. Parent is who the term is referring to, so eliminate that choice. In choice (A), vox populi means ‘voice of the people’.