Answer:
According to the question, Power = 1W and Zener breakdown voltage = 5V
We are given that the minimum voltage = 3V and the maximum voltage = 7V
$ {{I}_{\max }}=\frac{P}{{{V}_{z}}}=\frac{1}{5} $
$ {{I}_{\max }}=0.2A $
$ {{R}_{s}}=\frac{{{V}_{\max }}-{{V}_{\min }}}{{{I}_{\max }}}=\frac{7-3}{0.2} $
$ {{R}_{s}}=20\Omega $