Step 1: Concept
This relates to the properties of cosets in group theory.
Step 2: Meaning
$aH$ denotes a left coset of $H$ in $G$, which is the set $\{ah : h \in H\}$.
Step 3: Analysis
There is a one-to-one correspondence between $H$ and any of its cosets $aH$ defined by the map $f(h) = ah$.
Step 4: Conclusion
Because the map is a bijection, the number of elements (order) in the coset $aH$ is exactly the same as the order of the subgroup $H$, which is $m$.
Final Answer: (A)