b
When unregulated voltage is \(14\, {V}\) voltage across

zener diode must be \(10\, {V}\) So potential difference

across resistor \(\Delta {V}_{{Rs}}=4\, {V}\)

and \({P}_{\text {zener }}=2\, {W}\)

\({VI}=2\)

\({I}=\frac{2}{10}=0.2\, {A}\)

\(\Delta {V}_{{Rs}}={I} {R}_{{s}}\)

\(4 \times 0.2 {R}_{s} \Rightarrow {R}_{5}=\frac{40}{2}=20\, \Omega\)