Final answer:
A man is said to go 'stag' when he attends a social event without a female companion.
Step-by-step explanation:
A man goes 'stag' when attending a social event unaccompanied by a woman. This term is used to describe someone who is going alone to an event where couples or pairs are typically expected. For instance, if a man attends a wedding without a date, he can be said to be going stag. The term can apply to both casual and formal events. In contrast, the other options, such as 'bear', 'wolf', and 'lion', are not commonly used in this context.