Fill in the blank by selecting the appropriate word from the options…

The correct answer is Option (2) → into

Preposition of movement:

• Into shows movement from outside to the inside of a place or space.
• "In", "inside", "on" and "under" show where something is; "into" shows where something goes.
• This sentence: the car moved off the road and ended up inside the ditch, so it "fell into a ditch".
• Complete sentence: As Rohit was driving fast, his car hit the divider and fell into a ditch.
  Example: The boy jumped into the swimming pool.

Why the Other Options are Incorrect?

Option 1: on — Incorrect. On shows a position on the surface of something; a car falls into a ditch, not on it.
Example: The book is on the table.

Option 3: inside — Incorrect. Inside shows position within a closed place; with a verb of falling, the movement needs "into".
Example: The children are waiting inside the classroom.

Option 4: under — Incorrect. Under means below something; a car cannot fall below a ditch.
Example: The cat is sleeping under the bed.