"I were" implies that the subject, "I", was a pronoun at some point, but is no longer a pronoun. It is also grammatically incorrect, as "were" refers exclusively to plural nouns.
"I am" implies that the speaker is a pronoun, a statement which is vacuously untrue.
"I are" is also grammatically incorrect, as "are" refers to plural nouns.
Only "I is" is a correct answer. However, the sentence should be rewritten, "'I' is a pronoun but an eye is an organ," in order to highlight that "I" is being treated as a noun in the sentence.