According to Newton's second law of motion, \( F = ma \), where \( F \) is the net force, \( m \) is the mass, and \( a \) is the acceleration. If the net force \( F \) is a non-zero constant and the mass \( m \) increases, the acceleration \( a \) can be expressed as:
\[ a = \frac{F}{m} \]
If \( F \) is constant and \( m \) increases, then the acceleration \( a \) must decrease because the same force is being applied to a larger mass.
Therefore, the correct response is:
The acceleration decreases.