An alternator, also known as a synchronous generator, produces electrical power in a system. When assessing its operation, concepts such as power factor and the relationship between current and voltage are critical.

Power factor is defined as the cosine of the angle between the current and voltage in an AC circuit. It indicates how effectively electrical power is being converted into useful work output. A unity power factor means this angle is zero, implying that the current and voltage are perfectly aligned. In such cases, the current and voltage waveforms are in phase with each other.

Let's examine why "is in phase with terminal voltage" is the correct answer:

1. At unity power factor, the angle between the current and voltage is zero. This means:
   • PF = \cos(0^\circ) = 1, which indicates a perfect alignment.
2. Current being "in phase" implies that their peaks and zero crossings occur at the same time.
3. In this condition, all the power from the source is used effectively for work, with no reactive power involved.

While analyzing other options:

• Leads terminal voltage: This occurs in a capacitive load condition where the power factor is leading, not unity.
• Lags terminal voltage: This occurs in an inductive load where the power factor is lagging, again not unity.
• Is opposite to terminal voltage: This would imply a phase difference of 180°, which is not possible for power generation applications.

Therefore, under a unity power factor condition, the armature current of an alternator is indeed "in phase with terminal voltage".