What instrument can provide a direct measurement of power at the output of a transmitter?

To measure the power at the output of a transmitter, you need an instrument designed to handle Radio Frequency (RF) signals and display power in watts. An RF wattmeter is specifically designed to be connected directly in-line at the transmitter's output to measure forward (and often reflected) power. Other options are incorrect because they measure different properties. A field-strength meter measures relative signal strength over the air, not direct output power at the connector. An ammeter measures electrical current, and a wavemeter measures frequency or wavelength.