Loss Percent Calculation:
To find the loss percent, we need to compare the cost price (CP) and the selling price (SP) of the books.

Given:
Cost price of 12 books = Selling price of 15 books

Let's assume that the cost price of each book is 'x'.

So, the cost price of 12 books = 12x
And the selling price of 15 books = 15x

Loss Calculation:
The loss is calculated as the difference between the cost price and the selling price.

Loss = Cost price - Selling price

Since the cost price is greater than the selling price (as per the given condition), the loss will be positive.

Loss = 12x - 15x
Loss = -3x

Loss Percent Formula:
Loss percent is the ratio of the loss to the cost price, expressed as a percentage.

Loss Percent = (Loss / Cost price) * 100

Substituting the value of the loss, we get:

Loss Percent = (-3x / 12x) * 100
Loss Percent = -25%

Hence, the loss percent is -25%.

Explanation:
The negative loss percentage indicates a loss. In this case, the loss percent is -25%, which means that the selling price is 25% less than the cost price. The negative sign indicates that there is a loss.

Option B, which states 20% loss, is incorrect. The correct answer is option B, which states a 25% loss.