Step 1: Understand the force on the charged particle.
When the charge \( q \) enters the electric field between the parallel plates, the force \( F \) acting on it is given by:
\[
F = qE
\]
where \( E \) is the electric field between the plates. The electric field is related to the potential difference \( V \) and the distance between the plates \( d \) by:
\[
E = \frac{V}{d}
\]
Step 2: Apply Newton's second law.
The acceleration \( a \) of the charged particle is given by Newton's second law:
\[
a = \frac{F}{m} = \frac{qE}{m} = \frac{qV}{dm}
\]
Step 3: Conclusion.
Thus, the acceleration of the charged particle is \( \frac{qV}{dm} \), which corresponds to option (C).