Final answer:
The correct answer is option (C) CH₄. All the other options, NH₃, H₂O, and CN⁻, can act as Lewis bases.
Step-by-step explanation:
The correct answer is option (C) CH₄. All the other options, NH₃, H₂O, and CN⁻, can act as Lewis bases. A Lewis base is a substance that donates a pair of electrons to form a covalent bond.
NH₃ (ammonia) can act as a Lewis base by donating its lone pair of electrons to a Lewis acid, such as a metal ion.
H₂O (water) can act as a Lewis base by donating its lone pair of electrons to a Lewis acid, such as a proton (H⁺).
CN⁻ (cyanide ion) can act as a Lewis base by donating its lone pair of electrons to a Lewis acid, such as a metal ion.