Answer
$H_a: µ \gt 3$ matches with c;
the null hypothesis being $H_0: µ ≤ 3$
(see attached graph).
Work Step by Step
The alternative hypothesis $H_a:μ\gt3$ means that μ takes on all values greater than 3, and that is represented by graph (c).
The null hypothesis is the opposite: $H_o:μ\leq3$. See the graph attached.