Let m denote the strength of a 5 layer carbon tire and n denote the strength of an 8 layer carbon tire.
If we add one more layer to the 5 layer carbon tire, the strength becomes 120% of m or 
Let m denote the strength of a 5 layer carbon tire and n denote the strength of an 8 layer carbon tire.
If we add one more layer to the 5 layer carbon tire, the strength becomes 120% of m or 1.2*m. Similarly, by adding 3 more layers, the strength of the 8 layer carbon will be given by
n = m * 1.2 * 1.2 * 1.2 = 1.728
The ratio of strengths = n/m = 1.728/1 = 1.73
The correct answer is B.