A shopkeeper marks his goods 30% above his cost price but allows a discount of 10% at the time of sale. His gain is = ?

Answer: Option D

Let the cost price of goods = Rs. 100
Mark price of goods = Rs. 130 (30% discount)
Selling price of goods = 90% of 130 = Rs. 117
Gain = 117 - 100 = Rs. 17
$$\text{Gain}\%=\frac{17}{100}\times 100=17\%$$