Mohammed, a Zoo Keeper in Charge of the Lions' Den

Mohammed, a zoo keeper in charge of the lions' den, leaves the door to the lions' den unlocked after feeding time. A lion escapes and attacks a visitor to the zoo. Under which of the following categories of duty is liability most likely to be imposed upon Mohammed for his failure to act?

A) Special relationship
B) Voluntary assumption of responsibility
C) Statutory duty
D) Contractual duty