Select the option that expresses the opposite meaning of the underlined word. The explosive used is of my own formulation, and I can vouch for its efficiency.

Correct — A. The underlined word is vouch — to state on your own authority that something is true, to guarantee it.

Its opposite is invalidate: to take away the force of a claim, to show or declare that it does not hold.

Vouching for the explosive's efficiency backs the claim; invalidating it knocks the claim down. That is the reversal the question asks for → option (a).

Why the others are wrong

• (b)
  Maintain — Maintain, in its 'assert and go on asserting' sense, is a synonym of vouch, not its opposite. Its other sense — keep in repair — is irrelevant here.
• (c)
  Witness — To witness is to see an event or to testify to it. That puts it on the same side as vouch, supporting a claim rather than undoing it.
• (d)
  Certify — Certify is the closest synonym in the list — to attest formally, usually in writing. It is the trap for anyone who reads the instruction as 'nearest in meaning'.

This is an antonym item built the way SSC likes: three of the four options cluster as synonyms of the prompt word, and the fourth reverses it.

Certify, maintain and witness all mean 'to back a statement'. Only invalidate cancels one.

When you cannot place the prompt word confidently, group the options first. The odd one out in a synonym cluster is usually the key of an antonym item.

Key facts

• To vouch for something is to guarantee it or answer for its truth.
• Invalidate means to deprive of legal force or to show that something does not hold.
• Invalidate is built from in- (not) plus valid, so it names the reversal directly.

Common traps

• Answering with certify, the closest synonym, when the instruction says opposite
• Dismissing maintain on its 'keep in repair' sense and never testing its 'assert' sense

No directly related past PYQ was found.