Answer : The conjugate base of propanoic acid,
is, 
Explanation :
According to the conjugate acid-base pair theory, an acid is a proton donor and a base is a proton acceptor.
The balanced conjugate acid-base pair reaction will be,

In this reaction,
is a weak acid which donate a proton to
which is a weak base to form
as a conjugate base and
as a conjugate acid.
Hence, the conjugate base of propanoic acid,
is, 