If F=vCln(xb), then
F and v denote force and velocity, the dimensions of C are [MT]
x denotes distance, the dimensions of b are L−1
the dimension of vC never be same as F
the dimensions of x must be same as vcb
Logarithm has no dimensions.
[xb]=M0L0T0⇒[b]=L−1
Hence, option (b) is correct.