Concept:
This sentence deals with indicating the agency, channel, medium, or delivery mechanism by which an object (a gift) traveled from the sender to the recipient.
Let us systematically break down the prepositions provided in the choices:
• Option (A) to: "To" denotes direction or destination. A gift is sent *to* a person, but not *to* courier when courier is being utilized as the medium of transport.
• Option (B) in: "In" denotes containment inside a closed space or a specific state. A gift cannot be sent "in courier" unless we mean a physical vehicle, which would require an article (like *in a courier van*).
• Option (C) through: The preposition "through" is perfectly utilized to show the medium, intermediary system, agency, or channel through which a process or delivery is completed (e.g., *through post*, *through courier*, *through email*). It indicates that the courier company acted as the pipeline for the delivery.
• Option (D) with: "With" indicates accompaniment, possession, or using a physical tool directly in hand. It does not fit an organized external logistics service provider like a courier service. Note that "by courier" is also valid, but among the options provided, "through" is the correct choice.
Consequently, inserting "through" completes the sentence accurately: "The friend sent him a gift through courier."