c
Capacitance of parellel plate capacitor with a dielectric slab of thickness $t$ inside is given by:
$C =\frac{ A \in_0}{( d - t )+\frac{ t }{ K }}$, 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 $C =\frac{ A \in_0}{ d }$
Hence, capacitance of capacitor will remains unchanged.