Step 1: Wave speed on string:
For the second harmonic, the wavelength is the string length, $\lambda=0.5$ m. So $v_s=f\lambda=100\times0.5=50$ m/s.
Step 2: Use v = sqrt(T/mu):
$50=\sqrt{50/\mu}$, so $\mu=\dfrac{50}{2500}=0.02$ kg/m.
Step 3: Mass:
$0.02\times0.5=0.01$ kg $=10$ g. Option (C).
Final Answer:
The string wave speed is 50 m/s, so mu = 0.02 kg/m.
\[ \boxed{C} \]