Let A be area of each plate of the capacitor, d is the distance between the plates, then
![the variation of charge q versus potential difference V for capacitors C1 and C2](https://www.sarthaks.com/?qa=blob&qa_blobid=17340879284168881127)
The capacitance of the parallel plate capacitor
C = \(\frac{ɛ_oA}{d}\)
or C ∝ A (∵ d is the same for both capacitors)
Since area of C2 is double than C1
So C2 = 2C1
Slope of the q-V graph gives the capacitance.
i.e. Slope = \(\frac{q}{V}\) = C
Since slope of A is more than the slope of B so line A corresponds to C2 and B corresponds to C1.