Step 1: Understanding the Question:
The sentence says the government tried to do something to petrol prices, and failed. We need the phrase that means "bring down" or "reduce".
Step 2: Key Formula or Approach:
Read each phrasal verb by its everyday meaning and check which one naturally pairs with the word "prices".
Step 3: Detailed Explanation:
"Wipe off" suits things like debts, marks, or memories being erased entirely, not a price being lowered.
"Cap off" suits ending an event or achievement on a high note, unrelated to price control.
"Sweep aside" suits ignoring a criticism or objection, not adjusting a numeric value like a price.
"Force down" suits pushing a rate, level, or price lower through pressure or policy, which matches "petrol prices" perfectly.
Only "force down" combines correctly with "prices" while keeping the intended meaning of the sentence.
Step 4: Final Answer:
"Force down" is the correct phrase, so option (d) is correct.