Answer
See answer.
Work Step by Step
From the diagram, $C_1$ and $C_2$ are in parallel. They form an equivalent capacitance of $C_{12}=C_1+C_2$.
From the diagram, $C_3$ and $C_4$ are in parallel. They form an equivalent capacitance of $C_{34}=C_3+C_4$.
These 2 combinations are in series with each other.
Find the equivalent capacitance:
$$\frac{1}{C_{ab}}=\frac{1}{C_{12}}+\frac{1}{C_{34}}=\frac{1}{ C_1+C_2}+\frac{1}{ C_3+C_4}$$
$$C_{ab}=\frac{( C_1+C_2)( C_3+C_4)}{ C_1+C_2+ C_3+C_4}$$
