Choice A is the correct answer. Total cost combines fixed and variable components.

1. Fixed cost: $4.99 base fee
2. Variable cost: $0.75 per mile for $d$ miles = $0.75d$
3. Total: $4.99 + 0.75d$
4. Constraint: "Under $12" means $< 12$: $0.75d + 4.99 < 12$

?�� Strategic Tip: "Under" means strictly less than ($<$), while "at most" means less than or equal to ($\leq$).

Choice B is incorrect because it subtracts the base fee instead of adding it. Choice C is incorrect because it treats the base fee as the per-mile rate and vice versa. Choice D is incorrect because it uses $>$ instead of $<$, representing costs exceeding $12 rather than staying under.