d
When connected in parallel \(C _{ eq }= C _{1}+ C _{2}\)

When in series

\(C _{ eq }^{\prime}=\frac{ C _{1} C _{2}}{ C _{1}+ C _{2}}\)

\(C _{1}+ C _{2}=\frac{15}{4}\left(\frac{ C _{1} C _{2}}{ C _{1}+ C _{2}}\right)\)

\(4\left( C _{1}+ C _{2}\right)^{2}=15 C _{1} C _{2}\)

\(4 C _{1}{ }^{2}+4 C _{2}^{2}-7 C _{1} C _{2}=0\)

dividing by \(C _{1}{ }^{2}\)

\(4\left(\frac{ C _{2}}{ C _{1}}\right)^{2}-\frac{7 C _{2}}{ C _{1}}+4=0\)

Let \(\frac{ C _{2}}{ C _{1}}= x\)

\(4 x^{2}-7 x+4=0\)

\(b ^{2}-4 ac =49-64<0\)

No solution exits