Fill in the blank with the correct auxiliary verb from the choices given:
How _______ I tell the future? It is always uncertain.

Concept: Auxiliary verb: The verbs are (am, is, was, etc), have, and do when used with ordinary verbs to support passive forms, tenses, questions, and negative sentences, are called auxiliary verbs. It's also called a helping verb.

• Modal verbs are words that indicate suggestion, advice, ability, order, and advice.
• Should: The word ‘should’ can be used as a modal verb or helping verb which is used before ordinary verbs and used to give suggestions or advice.
• Is: The word ‘is’ can be used as a helping verb to show the present tense and with a singular noun.
• Can: Can is used to show the ability of a person. Here the word 'can' is the correct auxiliary verb because it is making complete sense. 
• Has: The word ‘has’ is also a helping verb. But it is not suitable to fill in the blank.
• Hence, option C is the correct option.