Choose the most appropriate word to complete the following sentence._…

The correct answer is Option (2) → While

Conjunction of time "while":

• While means during the time that. It introduces a longer action already in progress, usually in the past continuous (was sitting).
• The main clause gives a short event that happened in the middle of that action, usually in the simple past (stung).
• Clue word: the past continuous "was sitting", an action in progress.
• This sentence: he "was sitting" in the garden (longer action) when the scorpion "stung" him (sudden event), so "While" joins them.
• Complete sentence: While he was sitting in the garden, a scorpion stung him.
  Example: While I was reading, the lights went out.

Why the Other Options are Incorrect?

Option 1: After — Incorrect. After places the sting once the sitting was over; the continuous "was sitting" shows the action was still going on.
Example: After he had finished dinner, he went for a walk.

Option 3: Before — Incorrect. Before places the sting earlier than the sitting; that does not fit an action in progress.
Example: Switch off the fan before you leave.

Option 4: Till — Incorrect. Till means up to the time when and marks the end point of an action; "Till he was sitting" makes no sense.
Example: Wait here till I come back.

Contrast: "while" sets a sudden event inside an action in progress; "after", "before" and "till" place the event later than, earlier than or at the end of that action.