d
(d) Mass per unit length \(m = \frac{{2 \times {{10}^{ - 4}}}}{{0.5}}kg/m\)

  \( = 4 \times {10^{ - 4}}kg/m\)

Frequency of \(2^{nd}\) harmonic \({n_2} = 2{n_1}\)

\( = 2 \times \frac{1}{{2l}}\sqrt {\frac{T}{m}} \)\( = \frac{1}{{0.5}}\sqrt {\frac{{20}}{{4 \times {{10}^{ - 4}}}}} = 447.2Hz\)