Explanation:

- The shopkeeper buys mangoes at Rs 20/kg.

- He sells them at Rs 30/kg.

- However, he gives only 800 g instead of 1 kg for the same price.

- Effective selling price per kg becomes $\text{Rs }(30\times \frac{1000}{800})=\text{Rs }37.5$.

- The profit per kg is $\text{Rs }(37.5-20)=\text{Rs }17.5$.

- To find the profit percentage, use:

```

Profit Percentage = \left( \frac{\text{profit per kg}}{\text{Cost price per kg}} \right) \times 100

```

- So,

```

Profit Percentage = \left( \frac{17.5}{20} \right) \times 100 = 87.5\%

```

- ?? The correct answer is Option 4: 87.5%.