The correct answers are What the elusive thief left behind was a single red rose. and The inspector will aggressively pursue whoever tampered with the evidence.

A nominal relative clause functions as a noun phrase (acting as a subject, object, etc.) and contains its own antecedent within the relative pronoun. "What" means "the thing that," and "whoever" means "the person who."

Incorrect options:

• "The single red rose that the thief left behind..." contains a standard relative clause modifying the head noun "rose."

• "The inspector asked what the elusive thief had left behind" contains an interrogative noun clause (an embedded question), not a nominal relative clause.