Help the eccentric professor complete his rather alarming report to the safety committee.

The highly explosive chemical compound _____ yesterday has mysteriously vanished from the laboratory.

The correct answer is with which I was experimenting.

The verb phrase is "experiment with." In formal writing, we shift the preposition to the beginning of the relative clause ("with which").

Watch out for the sneaky double preposition trap! If you move "with" to the front, you cannot leave another "with" at the end of the clause.