Answer
(a) increase
(b) II
Work Step by Step
(a) Let the capacitance of each capacitor be $ C_1=C_2=C_3=C $
The two capacitors are connected in parallel, so the equivalent capacitance is $ C_{12}=C_1+C_2=2C $
Now the third capacitor is connected to it; the equivalent capacitance is $ C_{effective}=C_{12}+C_3=2C+C=3C $
Thus, the equivalent capacitance is increased.
(b) We know that the best explanation is option (II) -- that is, adding a capacitor in parallel will increase the total amount of charge stored, and hence increase the equivalent capacitance.
