Correct : a

Eliminating wrong options:
"It is myself who is responsible for this fiasco." - Incorrect. "Myself" cannot be used as a subject pronoun. It is a reflexive pronoun and can only be used when the subject is already "I".
"It is I who is responsible for this fiasco." - Incorrect. The relative pronoun "who" refers to "I" which is first person singular, so the verb must be "am" not "is".
"It is I who are responsible for this fiasco." - Incorrect. "Are" is used for plural or second person. Since "I" is first person singular, "am" is correct.
"It is I who am responsible for this fiasco." - Correct. "I" is the proper subject pronoun after "It is", and "who am" correctly agrees with the first person singular "I".
The grammatically correct sentence is "It is I who am responsible for this fiasco." (Option A)