Correct!

Why should you check over your shoulder before you change lanes in busy, slow-moving traffic?

To avoid having to give a signal
Mirrors do not cover blind spots
Correct answer
So traffic ahead will make room for you
So your balance will not be affected

DVSA theory test explanation

Before changing lanes, make sure there's a safe gap to move into. Looking over your shoulder allows you to check the area not covered by your mirrors, where a vehicle could be hidden from view. It also warns following drivers that you want to change lanes.