c
Capacitance of parellel plate capacitor with a dielectric slab of thickness \(t\) inside is given by:

\(C =\frac{ A \epsilon_0}{( d - t )+\frac{ t }{ K }}\), where \(K\) is the dielectric constant.

When aluminium sheet of negligible thickness is inserted, then thickness of slab \(t \rightarrow 0\) and value of \(K\) for metal (conductor) will be \(\infty\)

Therefore \(C =\frac{ A \epsilon_0}{ d }\)

Hence, capacitance of capacitor will remains unchanged.