I am the shortest complete sentence in the English language. What am I?

“A”

The letter 'A' is not a complete sentence; it is just a single letter and doesn't express a complete thought.

“I”

The word 'I' alone is not a complete sentence because it lacks a verb or a predicate to form a complete thought.

“It is”

While 'It is' is a complete sentence, it is longer than 'I am' and therefore does not fit the criteria of being the shortest complete sentence.