To know when to shift gears, you should monitor which instrument on the dash?

Answer: A — Tachometer

Shifting timing comes from engine speed. The tachometer displays engine RPM, so you can keep the engine in its efficient power range. As you accelerate, shift up before you push the RPM into the red zone, staying within the RPM band where the engine produces good power. If RPM drops too low, you’d downshift to maintain adequate engine torque. The speedometer, fuel gauge, and odometer don’t tell you how hard the engine is working, only speed, fuel level, and distance, respectively. So the tachometer is the tool to know when to shift.