Question

Always use significant figure rules. Remember that these rules apply to all numbers that are measurements.

Answer 1

I'll assume $\rho=1.8*10^{-6}$ (resistivity of the wire), I'm pretty sure this is the case given that the units you provided go accordingly with those of resistivity.

For a wire with cross-sectional area $A$, length $l$, and resistivity $\rho$, then its resistance $R$ is given by:
$R= \frac{L\rho}{A}$

So, for your example:
$R= \frac{L\rho}{A}=\frac{(90)(1.8*10^{-6})}{.02}=.0081=8.1*10^{-3}$

Hence, the answer is C.