Answer
In case (c), the acceleration does not change.
Work Step by Step
Newton's 2nd law states that $$\sum F=ma$$
(a) Because $\sum F=F_1+F_2+ ...+F_n$, if all these forces double, $\sum F$ will double and $a$ will double as a result.
(b) Following Newton's 2nd law, $\sum F$ acted on the object doubles will lead to $a$ doubling.
(c) Both $\sum F$ doubling and $m$ doubling cancel out each other so $a$ stays the same.
(d) If $m$ is reduced by half and $\sum F$ remains the same, from Newton's 2nd law, $a$ has to double to cater for the loss of $m$.
So (c) is the exception.
