The correct answer is: A. spring.
A spring is a mechanical device that stores energy when it is compressed or stretched. When the spring is released, it converts this stored energy into kinetic energy, which can be used to power a variety of devices. In a portable instrument, the spring is often used to provide the controlling torque. This torque is what keeps the instrument from moving around when it is being used.
The other options are incorrect because they do not provide the controlling torque in a portable instrument. Gravity is a force that pulls objects towards the center of the Earth. Eddy currents are electric currents that flow in a circular motion. Neither of these forces provides the controlling torque in a portable instrument.
In conclusion, the correct answer to the question “In a portable instrument, the controlling torque is provided by” is A. spring.