Solution

Class C addresses start with bits $110$.

So, first $3$ bits are fixed.

Remaining bits are

$32-3=29$

Total class C addresses are

$2^{29}$

Total IPv4 addresses are

$2^{32}$

So, fraction of class C address space is

$\dfrac{2^{29}}{2^{32}}=\dfrac{1}{8}$

Now,

$\dfrac{1}{8}\times 100=12.5%$