One of the simplest ways is to find the last number as it isn't a coefficient of X
You can find it by multiplying the 2 numbers in the bracket without X. E.G the 1 from (3x+1) and the 3 from (x+3)
so we're looking for 3 as it isn't a coefficient of X
for A, (1x3)=3
For B (2x2)=4
for C (2x3)=6
For D (1X2)=2
so the answer is A