The sentence in question, "______ on!" is an imperative sentence. Imperatives often end with an exclamation, and give an order or command.
The imperative here is a quick and sudden reaction by the mother wolf.
Option A: The verb in such sentences (called as imperative verbs) are created using the root of the verb, e.g. 'to hold', where 'hold' is the root or base.
Hence, the correct verb is 'hold', and option A is right.
Options B, C and D: 'Was holding' refers to a past continuous action. It is not an appropriate form for imperatives.
'Holds' refers to the simple present, and describes a general truth or fact.
'Held' refers to an action that was completed in the past.
These verb forms are used in assertive sentences, or interrogatives. They declare an action, or ask whether the action happened. They are not used in imperatives.
Hence Options B, C and D are incorrect.