Fill in the blank in the following sentence with the most appropriate…

The correct answer is Option (4) → had been sawing

Past perfect continuous (an action in progress before a past event):

• Form: had been + verb-ing (had been sawing).
• Use: for an activity that had been going on for some time up to a moment in the past, when something else happened.
• Deciding clue: "when it started to rain" (simple past) marks the moment that broke into the work.
• This sentence: the woodcutter's sawing was already going on when the rain began, so "had been sawing" fits.
• Complete sentence: The woodcutter had been sawing the log when it started to rain.

Why the Other Options are Incorrect?

Option 1: sawed — Incorrect. The simple past shows a single completed action. With "when it started to rain" it would mean that he began sawing after the rain started, not that the work was in progress.
Example: He sawed the plank in half.

Option 2: saws — Incorrect. "Saws" is the simple present, used for habits and general facts. It clashes with the past tense "started".
Example: He saws wood every morning.

Option 3: saw — Incorrect. "Saw" is the base form of the verb "saw" (to cut) or the past tense of "see". As the past of see, the sentence would say that the woodcutter looked at the log.
Example: I saw a snake in the garden.

Contrast: the simple past "sawed" shows a single finished action. "Had been sawing" shows work already in progress when the rain began.