Given, Half-life of the reaction (t1∕2)=10min Initital concentration (a)=0.08M Concentration at time 't' i.e. (a−x)=0.01M As per relation of first order
t
t1∕2
=
log[a∕a−x]1
log[a∕a−x]2
=
log‌
0.08
0.01
log‌2
t
10
=
log‌8
log‌2
=
3‌log‌2
log‌2
=
3×0.3010
0.3010
t=3×10=30‌min. Hence, (b) is the correct option.