How to find the correct graph for variation of capacitance with time.?
Perhaps it will help with visualisation to consider the slightly simpler case of a rectangular cut-out, frozen in time with a length, x, of the cut-out between the plates, as below. Assume the depth of the cross-section is 1, so the total area of the top plate, for example, is \$A=L*1=L\$.

This can be modelled by the capacitor arrangement shown below the cross-section (perhaps if you imagine a fictitious thin conductor plate separating the two smaller rectangles it will convince you that they are two capacitors acting in series).

The question, as set, does not need mathematical analysis as it's just looking for qualitative judgement, however, it may be worthwhile going some way towards developing a mathematical model...

... using the formula for capacitance: \$C=\frac{\epsilon_r \epsilon_o  Area}{Distance}\$, the various capacitor values are: \$C_1=\frac{\epsilon_r \epsilon_o (L-x)}{D}\$; \$C_2=\frac{\epsilon_r \epsilon_o x}{D-d}\$; \$C_3=\frac{\epsilon_o x}{d}\$

Hence the overall capacitance is \$C=C_1 + \frac{C_2 C_3}{C_2+C_3}\$, and the effects of each term, as the dielectric moves through the plates, can be determined by letting x increase progressively from zero.

Not a particularly easy formula to evaluate, but it's easier than the triangular cut-out which results in a logarithmic relationship, as it involves integration of terms of the forms \$1/x\$ and \$1/(x+a)\$