d
(d) By using \({C_{air}} = \frac{{{\varepsilon _0}A}}{d},\) \({C_{medium}} = \frac{{{\varepsilon _0}A}}{{d - t + \frac{t}{K}}}\)
For \(K = \infty\) \({C_{medium}} = \frac{{{\varepsilon _0}A}}{{d - t}}\)
\(==>\) \(\frac{{{C_m}}}{{{C_a}}} = \frac{d}{{d - t}}\) \(==>\) \(\frac{{{C_m}}}{{15}} = \frac{6}{{6 - 3}}\) \(==>\) \({C_m} = 30\,\mu \,C\)