A written subcontract requires the general contractor's prior written approval before the subcontractor may assign the subcontract. The subcontractor assigns it anyway with no approval. The assignment is most likely:

Explanation

Parties may contractually restrict or condition the right to assign. When a subcontract expressly requires prior written approval for assignment, an assignment made without that approval violates the clause and is generally ineffective or constitutes a breach. Assignments are not always freely allowed when the contract says otherwise. Ignoring the clause does not create a novation, which needs everyone's consent, and cannot bind the owner in disregard of the agreed restriction.