b
As the bubble is rising steadily the net force acting on it will be zero
(Because of density of air the value of mg can be neglected)
So $B = F \Rightarrow \frac{4 \pi}{3} R ^{3} \rho g =6 \pi \eta Rv$
Putting $R =1\,mm =10^{-3}\,m$
$\rho=1.75 \times 10^{3}\,kg / m ^{3}$
$g =10\,m / s ^{2}$
$v =0.35 \times 10^{-2}\,m / s$
$\eta=\frac{10}{9}=1.11$ SI unit $=11$ poise $( CGS )$