Choose the correct pair from the given options to fill in the blanks.…

The correct answer is Option (1) → was evaluating, had disappeared

Past continuous and past perfect together:

• Past continuous (was/were + -ing) shows an action in progress at a past time; "while" usually introduces such an action.
• Past perfect (had + past participle) shows an action completed before another past action.
• Clue word: "while". This sentence:
  • she was in the middle of checking the answer sheets: was evaluating;
  • the paper went missing before she found out: had disappeared.
• Complete sentence: While she was evaluating the answer sheets, she found that one paper had disappeared from the room.
  Example: When we reached the station, the train had left.

Why the Other Options are Incorrect?

Option 2: had evaluated, disappeared — Incorrect.

• "While" needs an action in progress; "had evaluated" is a finished action.
• "Disappeared" does not show that the paper went missing before she found out.

Option 3: evaluated, disappeared — Incorrect. The simple past in both blanks shows neither the action in progress nor which event came first.

Option 4: had been evaluating, disappeared — Incorrect. "Disappeared" in the simple past does not place the disappearance before her discovery; the earlier action needs the past perfect.