This sentence is talking about one person, the pronoun "I". The only choice that goes with the word "I" is "am". "Is" and "has" goes with the pronouns "she" and "he". "Were" goes with a group of people, like "they".

This sentence is talking about one cat, we know this because the sentence says "the cat". Because we are only talking about one cat, "were" and "are" are not correct because we would use those words to talk about more than one cat. The word "am" is used with the pronoun "I", so that is not correct. The correct answer is "is".

"The cat is on the step."