Which instrument should be used to determine bank when the heading indicator shows a turn to the left?

The appropriate instrument to determine bank during a left turn, as indicated by the heading indicator, is the attitude indicator. The attitude indicator displays the aircraft's orientation relative to the horizon, effectively showing whether the aircraft is climbing, descending, or maintaining level flight, as well as the bank angle during turns.

In a turn, the attitude indicator allows the pilot to visually confirm the amount of bank angle being used. It provides crucial information about the aircraft's roll state, thus aiding in the management of the turn and ensuring that it is performed safely and effectively.

While the turn coordinator also indicates rate of turn and can show bank angle indirectly, the attitude indicator gives a more direct and comprehensive representation of the aircraft's orientation and bank angle, making it the ideal choice for confirming the bank during a turn. The other instruments listed, like vertical speed indicators and heading indicators, do not specifically measure bank angle in a meaningful way during turns.