b
Length of the steel wire, \(l=12 \,m\)

Mass of the steel wire, \(m=2.10\, kg\)

Velocity of the transverse wave, \(v=343\, m / s\)

Mass per unit length, \(\mu=\frac{m}{l}=\frac{2.10}{12}=0.175\, kg\, m ^{-1}\)

For tension \(T\), velocity of the transverse wave can be obtained using the relation:

\(v=\sqrt{\frac{T}{\mu}}\)

\(\therefore T=v^{2} \mu\)

\(=(343)^{2} \times 0.175=20588.575 \approx 2.06 \times 10^{4} \;N\)