Explanation:

- Let's say the cost price of the earphone is Rs \( x \).

- The percentage profit is also equal to \( x \).

- Selling Price (SP) = Cost Price (CP) + Profit

- Rs 2000 = \( x + \left(\dfrac{x \times x}{100}\right) \)

- \( 2000 = x + \dfrac{x^2}{100} \)

- \( 2000 \times 100 = 100x + x^2 \)

- \( x^2 + 100x - 200000 = 0 \)

- We solve this quadratic equation for \( x \).

- Solving \( x^2 + 100x - 200000 = 0 \) gives us possible cost prices.

- Checking options:

- Option 4 (Rs 400): If the cost price is Rs 400, then the profit is 400% which means the selling price = \( 400 + \frac{(400 \times 400)}{100} \).

- This calculation shows RS 2000 as the selling price.

So, the correct option is Option 4 (Rs 400).