The correct preposition to fill in the blank is: The meeting is scheduled at 1:30 pm.

Here's why:

• "At" is used to indicate specific times of the day: "at noon," "at 5 o'clock," "at breakfast time." This aligns with the context of scheduling a meeting for a specific time, 1:30 pm.
• "On" is typically used with days of the week or dates: "on Monday," "on Christmas Day."
• "In" is used for larger periods of time: "in the morning," "in the summer." It wouldn't be suitable for a specific point in time like 1:30 pm.
• "By" indicates a deadline or a point in time before which something needs to be done: "by 5 pm," "the report is due by Friday."