Please tell how it became 1/4.

Dear Student,
As mentioned in the question, applied force is halved, F' = F/2, and mass of the truck is doubled, M' = 2M, so it acceleration will be
$a=\frac{F\text{'}}{M\text{'}}=F\text{'}\times \frac{1}{M\text{'}}=\frac{F}{2}\times \frac{1}{2M}=\frac{F}{4M}=\frac{1}{4}.\frac{F}{M}$
Regards