Which is a primary task performed by a mega instrument?

A mega instrument, commonly known as a megohmmeter, is specifically designed to measure insulation resistance in electrical systems. This is crucial for ensuring that insulation materials used in wiring and electrical components are effective and do not allow current to leak, which could lead to equipment failure or electrical hazards. By applying a high voltage to the insulation and measuring the resistance, these instruments help assess the integrity of the insulation.

In the context of the other choices, while testing AC current, measuring voltage drop, and calculating power factor are important tasks in electrical work, they are not the primary functions of a mega instrument. These tasks typically require different equipment, such as multimeters or clamp meters, which are designed for those specific measurements rather than focusing on insulation resistance.