The correct answer is: A. Present Continuous.
The verb “is” is used as a helping verb in the present continuous tense to form the progressive aspect. The progressive aspect is used to describe actions that are happening at the moment of speaking or that are happening over a period of time. For example, “I am writing a letter” describes an action that is happening at the moment of speaking. “I have been writing a letter for an hour” describes an action that has been happening over a period of time.
The verb “am” is used as a helping verb in the present continuous tense to form the progressive aspect with the subject “I”. The verb “are” is used as a helping verb in the present continuous tense to form the progressive aspect with the subjects “you”, “he”, “she”, “it”, “we”, and “they”.
The past indefinite tense is used to describe actions that happened at a specific time in the past. The past continuous tense is used to describe actions that were happening at a specific time in the past. The past perfect tense is used to describe actions that had happened before a specific time in the past.
For example, “I wrote a letter yesterday” describes an action that happened at a specific time in the past. “I was writing a letter when you called” describes an action that was happening at a specific time in the past. “I had written a letter before you called” describes an action that had happened before a specific time in the past.