Answer
b. 1/2
Work Step by Step
The answer can be obtained by substituting the values given by the question into the mid-point formula for the price elasticity of demand
Price elasticity of demand $$=\frac{(Q2 - Q1)/ ((Q2 + Q1)/2)}{P2 - P1)/ ((P2 + P1)/2)}$$
$$=\frac{(90 - 110)/ ((90 + 110)/2)}{(12 - 8)/ ((8 + 12)/2)}$$
$$=1/2$$
$$= b.$$
