The correct option is (a) Covalent bond
Best explanation: This type of prodrug where the active ingredient and inert drug is bonded by the covalent bond comes under carrier-linked prodrug. They are generally esters or amide. These drugs and modified their lipophilicity to a great extent because of being attached to a carrier. The active drug is released by hydrolytic cleavage. The hydrolytic cleavage can be chemical or enzymatic.