The above question is looking very simple but there are many things to understand it. Mostly student just try to give the answer quickly and forget many mathematical things. Denominator is also an important thing.

Denominator

Let we have 1/2x. It clearly show that 2x is denominator. There is no need any parentheses around 2x. It is understood that 2x is denominator

i.e. 1/2x = 1/(2x) = (2x)^-1

This sort of notation is used especially with pi, ln, or e. We have never had to say 1/(2pi). We use to simply written as 1/2pi or 1/2e^2 without any parentheses.
Similarly in ab/cd, ab is nominator and cd is denominator. Without any parentheses and its mean (ab)/(cd) and parentheses no need to use here.

Similarly in given question 2(1+2) is denominator. No need to write it as {2(1+2)}