CORRECT!

The ball of the inclinometer in a turn coordinator is the only instrument that indicates the quality (coordination) of a turn. Recalling your primary flight instruction, there are slipping turns (in which the aircraft slides to the inside of the turn) and skidding turns (aircraft skids to the outside of the turn). Both conditions are remedied with proper use of the rudder.

"Step on the ball" is the familiar direction you probably heard from your flight instructor if he or she noticed an uncoordinated flight condition. If the ball in the inclinometer is out to the left during a right turn (a skidding turn), more left rudder pressure is needed to center the ball and stop the skid. Likewise but opposite for a slipping turn: if the ball is out to the right during a right turn (a slipping turn), more right rudder pressure is needed to control yaw and stop the slip.

Answer a) is incorrect because the heading indicator and compass provide direction or heading information only and give no indication of the turn quality. Answer b) is incorrect because the attitude indicator indicates pitch and bank angles, but not turn quality.