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. You can use it to describe both events that are happening in the present – right now, while you are talking about something, or in the future – something that may or will happen later on. So, we would be using present continuous tense for the given blank. Since it is in the present, "was", "has been" and "will be" is wrong as they are past, present perfect and future verbs.
Now, in continuous tense, the verb always ends with an '-ing',
Option C is correct.