Answer:
b. NH₃.
Explanation:
Base is the substance that can produce OH⁻ ions in the solution.
So, NaOH, KOH, and NH₃ are bases because they can produce OH⁻ ions in the solution.
But, K₂O is not a base, it can not produce OH⁻ ions in the solution.
Strong bases are bases that can completely ionized in the solution and produce OH⁻ ions.
NaOH and KOH are strong bases because they can completely ionized in the solution and produce OH⁻ ions.
Weak bases are bases that can not completely ionized (ionized partially) in the solution and produce OH⁻ ions.
NH₃is a weak base hat can not completely ionized (ionized partially) in the solution and produce OH⁻ ions.
So, the right choice is: NH₃.