Which of the following is not a Lewis acid?

Explanation:

Lewis acid is considered one type of molecule in which the central atom should contain vacant or empty orbital which accept electrons from outside, i.e should act as an electron acceptor.

In SiF₄, central atom silicon has a vacant d orbital which accepts  from outside and behaves as a Lewis base.

In FeCl₃ , central atom iron has an incomplete octet. Therefore  it has a vacant orbital to accept electrons and behave as Lewis acid.

In BF₃ , the central atom boron has empty P orbitals which accept electrons from outside and behave as a Lewis acid.

C₂H₂ , the carbon atom completes its octet therefore no vacant or empty orbital present on it to accept electrons. So it's not act as a Lewis acid.

Final Answer:

Option D) C₂H₂