The prefix that means normal or good is pro (option d). In various contexts, ‘pro’ denotes support or favor for something, often implying a positive stance or quality. For example, in terms like ‘proactive’ or ‘promote’, it indicates a positive action or endorsement.
On the other hand, ‘eu’ (option a) also implies good, particularly in biological and medical terminology, as in ‘eukaryote’, which refers to organisms with complex cells. However, when considering the most direct meaning related to normal or good in a general context, ‘pro’ is the most fitting choice.