c
\(v_{1}=50 km / hr\) due north

\(v_{2}=50 km / hr \text { due west }\)

\(-v_{1}=50 km / hr \text { due south }\)

Magnitude of change in velocity

\(=\left|\vec{v}_{2}-\vec{v}_{1}\right|=\left|\vec{v}_{2}+\left(-\vec{v}_{1}\right)\right|\)

\(=\sqrt{v_{2}^{2}+\left(-v_{1}\right)^{2}}\)

\(=\sqrt{(50)^{2}+(50)^{2}}=70.7 km / hr\)

\(\vec{v}=70.7 km / hr\) along south-west direction