Step 1: Understanding the Concept:
$(a,b) = c$ means $c$ is the greatest common divisor of $a$ and $b$.
Step 2: Detailed Explanation:
By definition of GCD, $c$ divides both $a$ and $b$. So both $\dfrac{a}{c}$ and $\dfrac{b}{c}$ are integers, i.e.\ $c \mid a$ and $c \mid b$.
Option (A) $c \mid a$ is therefore correct.
Step 3: Final Answer:
$c \mid a$ is true.