Step 1: Recall the three stage idea.
Traditional Indian learning is said to move through three steps. First comes shravana, listening to the teacher. Next comes manana, thinking over what was heard until doubts are gone. Last comes nididhyasana, steady meditation until the truth is lived and not just known.
Step 2: Find the source verse.
The verse that says the self should be seen, heard, reflected on and meditated on is spoken by Yajnavalkya to Maitreyi. It sits in the Brhadaranyaka Upanishad, in sections 2.4 and 4.5. This is the root of the three stage path.
Step 3: Test the other texts quickly.
Katha is about death, Nachiketa and the nature of the self. Isha is about seeing the divine in everything and joining action with knowledge. Mandukya is about Om and the four states of mind. None of them states the three stage method of hearing, reflecting and meditating.
Step 4: Match with the options.
Option 2 names the Brhadaranyaka Upanishad. Options 1, 3 and 4 name texts whose main themes are different, so they are left out.
Step 5: Second check.
Many students pick the Katha Upanishad because it is the most famous teaching story. But fame is not the test here. The test is the three part method. A good way to remember it is the line 'hear it, think it, live it'. That line comes from the Brhadaranyaka passage, so the answer stays option 2 even after a second look. Also remember that the Mandukya Upanishad counts four states, so the number three does not match it either.
Final Answer:
The Brhadaranyaka Upanishad is the source of the three stage path, so option 2 is correct.
\[\boxed{\text{Option 2}}\]