The present continuous tense is formed with the subject plus the present particle form (-ing) of the main verb and the present continuous tense of the verb to be: am, is, are.
In this particular sentence, we can understand that the question is based on something which is happening. The present continuous tense is a grammatical tense that can be used to describe when an action happened, or may happen. So, we would be using present continuous tense for the given blank.
Now, in continuous tense, the verb always ends with an '-ing', thus this would eliminate option C. Since, it is the present, we would be using "am" instead of "was"
This makes, A the correct answer.