An infant depends ___ its mother for food.

The correct preposition to use with the verb "depend" when indicating reliance or needing support is "on" or "upon." Therefore, "An infant depends on its mother for food" is grammatically correct and commonly used.

• "at" (A) is incorrect; "depend at" is not a standard English idiom.
• "upon" (C) is also grammatically correct and synonymous with "on," but "on" is often the more common and direct choice.
• "to" (D) is incorrect; "depend to" is not standard English usage in this context.