Right option is (a) R = (E * W * T) / (I * L)
For explanation: Specific resistance is measured in the length direction of the brush, since resistance in the direction of width or thickness may be considerably different. For, E = voltage drop over length L, I = amps of current passed through the sample, W = width of sample, T = thickness on sample, L = that portion of the length, over which the voltage drop E is measured, R is calculated by R = (E * W * T) / (I * L).