I'll assume [tex]\rho=1.8*10^{-6}[/tex] (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 [tex]A[/tex], length [tex]l[/tex], and resistivity [tex]\rho[/tex], then its resistance [tex]R[/tex] is given by:
[tex]R= \frac{L\rho}{A} [/tex]
So, for your example:
[tex]R= \frac{L\rho}{A}=\frac{(90)(1.8*10^{-6})}{.02}=.0081=8.1*10^{-3}[/tex]
Hence, the answer is C.
