Let's denote:

• x as the number of pairs of brown socks in the original order.
• p as the price of a pair of brown socks.

Given that:

• The price of a black pair is three times that of a brown pair, so the price of a black pair is $3p$.
• The person ordered 5 pairs of black socks and x pairs of brown socks.
• The clerk mistakenly interchanged the number of black and brown pairs, increasing the bill by 100%.

Original Total Cost: The total cost of the original order is: $5 \cdot 3p + x \cdot p = 15p + xp$

Incorrect Total Cost: After the interchange, the total cost becomes: $x \cdot 3p + 5 \cdot p = 3xp + 5p$

Condition Given: The bill increased by 100%, meaning the incorrect total cost is double the original total cost: $3xp + 5p = 2 \cdot (15p + xp)$

Solving the Equation:

1. Expand both sides: $3xp + 5p = 30p + 2xp$

2. Simplify the equation: $3xp - 2xp = 30p - 5p$ $xp = 25p$

3. Divide both sides by $p$ (assuming $p eq 0$): $x = 25$

Conclusion: The number of pairs of brown socks in the original order was 25.

Therefore, the correct answer is D. 25.