Select the most appropriate preposition to fill in the blank:

While exploring the university library's basement, the researcher stumbled ____ a rare manuscript hidden behind a row of old books.

The correct answer is upon.

Detailed Explanation:
The sentence requires a preposition that completes the phrasal verb meaning "to find or discover something unexpectedly or by chance."

The phrasal verb stumble upon (or stumble across) means to discover something by chance or unexpectedly. In the context of the sentence, the researcher unexpectedly found a rare manuscript hidden behind old books while exploring the library's basement, making upon the correct choice.

Option Breakdown:
1. over: "Stumble over" means to physically trip over an object (e.g., "stumble over a loose wire").
2. upon: Correct. Means to unexpectedly discover or come across something.
3. along: "Stumble along" refers to walking or moving forward unsteadily.
4. into: "Stumble into" typically means entering a place unsteadily or becoming involved in a situation unexpectedly, rather than finding a hidden item.