Identify the part which has an error in it.
She discovered / to her horror / that she swallowed a fly / while drinking tea.

Correct option is C

Error part: (c) that she swallowed a fly
What is wrong (in detail):
• Main verb is discovered (past), so the action that happened during drinking should be in Past Continuous to show an ongoing action.
• “swallowed” (simple past) is less appropriate here; expected: was swallowing / had swallowed depending on meaning.
Most suitable correction (with given context):
• She discovered to her horror that she had swallowed a fly while drinking tea.
Grammatical Rule (English):
• Use Past Perfect for the earlier past action discovered later (discover → had swallowed).
व्याकरणिक नियम (Hindi):
• जब कोई घटना बाद में पता चले (discover) और घटना पहले हो चुकी हो, तो पहले वाली क्रिया के लिए Past Perfect (had + V³) आता है।
Hence, option (c) is the correct answer.