Identify the grammatically correct sentence:

The correct option is: It is I who am responsible for this fiasco.

To understand why this sentence is grammatically correct, we can analyze it step-by-step:

1. Choice of Pronoun ("I" vs. "myself"):
In the cleft sentence structure "It is [pronoun]...", the pronoun functions as a predicate nominative. Therefore, the subjective pronoun I is grammatically correct, whereas the reflexive pronoun myself is incorrect because reflexive pronouns require an antecedent within the same clause to refer back to (e.g., "I did it myself").

2. Antecedent and Relative Pronoun Agreement:
In the relative clause starting with "who", the relative pronoun who refers directly back to the subject pronoun I. In grammar, the pronoun I is the antecedent of who.

3. Subject-Verb Agreement inside the Relative Clause:
A verb in a relative clause must agree in person and number with the relative pronoun's antecedent. Since the antecedent is I (which is first-person singular), the verb following "who" must also be in the first-person singular form.

The first-person singular present tense of the verb "to be" is am (e.g., "I am"). Using "is" (third-person singular) or "are" (second-person or plural) violates this agreement rule.

Combining these rules, "who am" is the grammatically correct sequence, making "It is I who am responsible for this fiasco" the only correct option.