He is ___ his study table:

The correct answer is "at". The preposition "at" is used to indicate a specific location or point, often when someone is situated near or working at a piece of furniture like a table or desk. "At his study table" means he is positioned there for the purpose of studying.

• "on" would imply he is physically on top of the table, which is incorrect for a person.
• "in" suggests being inside something, like a room or a box, not typically used for being next to a table.
• "by" means next to, which is plausible but "at" is more common and precise for indicating presence at a workstation.