Answer: Option (C) is the correct answer.
Explanation:
According to Arrhenius, bases are the species which when dissolved in water will give hydroxide ions, that is,
.
For example, 
Arrhenius acids are the species which when dissolved in water will give hydrogen ions, that is,
.
For example, 
In water, when an acid loses a hydrogen ion then the specie formed is known as conjugate base.
Here,
is the conjugate base of
.
Similarly, species which accept the hydrogen ion result in the formation of conjugate acid.
Hence,
is the conjugate acid of
.
Thus, we can conclude that
is the conjugate base.