\(V_A-V_B=8 \text { volt }\)

Current through zener diode,

\(i =\frac{ P }{ V }=\frac{1.6 W }{8 V }=0.2\,A\)

\(V _{ C }- V _{ A }=10-8 \text { volt }\)

\(\therefore R =\frac{ V _{ C }- V _{ A }}{ i }=\frac{2 V }{0.2 A }=10\,\Omega\)

[Note : A zener diode can regulate only if input voltage is \(\geq\) zener breakdown voltage the range of input voltage should be \(8\) to \(10\,V\) so that output voltage remains constant \(=8\,V\) ]