开启转向灯规定 mandates turning on your turn signal when making turns or changing lanes (whether legal or illegal) to alert the incoming or traffic behind. The main purpose of this regulation is to avoid collisions (追撞 in this case).
In other words, if this driver didn't turn on the turn signal, he might have received two citations 1. illegally changing lanes, and 2. changing lanes without flashing the turn signal.
Now let's move on to the question:
"....以下说法正确的是什么" - According to the photo, which of the descriptions below is correct (in the sense of matching the photo shown)?
B、压实线变更车道 (True, wheel on the solid median strip while intending to change lanes - the flashing turn signals and the direction of the front tire tell the intention)
C、未按规定开启转向灯 (False, the yellow turn signals are on as required for making turns and changing lanes)