Concept:
For the reaction
\[
2\mathrm{HI}\rightleftharpoons \mathrm{H_2}+\mathrm{I_2},
\]
the equilibrium constant is
\[
K_c=\frac{[\mathrm{H_2}][\mathrm{I_2}]}{[\mathrm{HI}]^2}.
\]
Step 1: Determine equilibrium moles.
Initially,
\[
\mathrm{H_2}=15,\qquad
\mathrm{I_2}=5.2,\qquad
\mathrm{HI}=0
\]
Since \(10\) moles of HI are formed,
\[
\mathrm{H_2+I_2\rightarrow 2HI}
\]
Therefore,
\[
x=\frac{10}{2}=5
\]
Equilibrium moles:
\[
\mathrm{H_2}=15-5=10
\]
\[
\mathrm{I_2}=5.2-5=0.2
\]
\[
\mathrm{HI}=10
\]
Step 2: Calculate \(K_c\).
\[
K_c=\frac{10\times0.2}{10^2}
=\frac{2}{100}
=2\times10^{-2}
\]
Step 3: Final conclusion.
\[
\boxed{K_c=2\times10^{-2}}
\]
Hence, the correct option is \(\boxed{(C)}\).