In this sentence, "It is I" is the subject, and "am" is the correct verb for "I." This is a formal construction, and the verb "am" correctly agrees with the subject "I."
The other options contain errors in subject-verb agreement or incorrect pronoun usage.
Option (A): "It is I who am responsible for this fiasco."
This is grammatically correct because the subject "I" requires the verb "am."
Option (B): "It is myself who is responsible for this fiasco."
The reflexive pronoun "myself" is incorrect. It should be "I" instead.
Option (C): "It is I who is responsible for this fiasco."
This sentence is incorrect because the subject "I" requires "am" and not "is."
Option (D): "It is I who are responsible for this fiasco."
This is incorrect because "are" is plural, and "I" is singular.