A transformer having the efficiency of 90% is working on a 200V, 15A supply. The secondary current is 13.5A. Find the secondary voltage.

Given:
Efficiency (η) = 90% = 0.9
Primary voltage (V_p) = 200 V
Primary current (I_p) = 15 A
Secondary current (I_s) = 13.5 A

The efficiency of a transformer is given by:
η = (Output power) / (Input power)
η = (V_s * I_s) / (V_p * I_p)

We need to find the secondary voltage (V_s). We can rearrange the efficiency formula to solve for V_s:
V_s = (η * V_p * I_p) / I_s

Substituting the given values:
V_s = (0.9 * 200 V * 15 A) / 13.5 A
V_s = 2700 / 13.5
V_s = 200 V

Therefore, the secondary voltage is 200V. However, this isn't one of the options. Let's re-examine the problem statement. It's possible there's an error in the provided question or options. The calculation above is correct based on the provided data and standard transformer efficiency formula.