The subject in the phrase "It is I" is "I," and the correct verb to use with "I" is "am." This is a formal construction, and the verb "am" correctly agrees with the singular subject "I." Other choices present errors in subject-verb agreement or improper pronoun usage.
Option (A): "It is I who am responsible for this fiasco." This sentence is grammatically sound as the subject "I" necessitates the verb "am."
Option (B): "It is myself who is responsible for this fiasco." The use of the reflexive pronoun "myself" is erroneous. The correct pronoun is "I."
Option (C): "It is I who is responsible for this fiasco." This sentence is incorrect because the subject "I" requires the verb "am," not "is."
Option (D): "It is I who are responsible for this fiasco." This is incorrect as "are" is a plural verb, and "I" is a singular subject.