Given:
Cost price of 20 articles = Selling price of 25 articles

To find:
Percentage profit or loss made by the merchant

Let's assume the cost price of each article is 'x' and the selling price of each article is 'y'.

Calculation:
Cost price of 20 articles = 20x
Selling price of 25 articles = 25y

According to the given information,
20x = 25y

To find the profit or loss percentage, we need to compare the selling price with the cost price. If the selling price is higher than the cost price, it is a profit. If the selling price is lower than the cost price, it is a loss.

Profit = Selling price - Cost price
Loss = Cost price - Selling price

Since the cost price of 20 articles is equal to the selling price of 25 articles, we can write it as:
25y - 20x = 0

Dividing the equation by 20, we get:
(25/20)y - x = 0
(5/4)y - x = 0

Now, let's calculate the profit or loss percentage:
Profit percentage = (Profit/Cost price) * 100
Loss percentage = (Loss/Cost price) * 100

Substituting the values from the equation (5/4)y - x = 0, we get:
Profit percentage = ((5y/4 - x)/x) * 100
Loss percentage = ((x - 5y/4)/x) * 100

Simplifying further,
Profit percentage = (5y/4x - x/x) * 100
Loss percentage = (4x/4x - 5y/4x) * 100
Profit percentage = (5y - 4x)/4x * 100
Loss percentage = (4x - 5y)/4x * 100

As we can see, the profit percentage and loss percentage are different.

Comparing the equations with the given options:
a) 25% loss - This option implies a loss percentage of 25.
b) 25% profit - This option implies a profit percentage of 25.
c) 20% loss - This option implies a loss percentage of 20.
d) 20% profit - This option implies a profit percentage of 20.

Since the equations for profit percentage and loss percentage do not match any of the given options, none of the options are correct.

Therefore, the correct answer is none of the above options.