Complete the project manager's slightly accusatory email regarding the disastrous new company mascot.

Please stop blaming the marketing interns; it is I who _____ responsible for approving the giant neon platypus.

The correct answer is am.

In an it-cleft sentence where the focused element is a subject pronoun ("I"), the verb in the relative clause must agree with that pronoun.

Even though "who is" sounds very natural in casual speech, formal grammar requires the verb to match "I" (I am -> It is I who am).