What is the perimeter of a triangle with sides measuring 3, 4, and 5?

To find the perimeter of a triangle, you simply add the lengths of all its sides together. In this case, the sides of the triangle measure 3, 4, and 5 units. By performing the addition:

3 (first side) + 4 (second side) + 5 (third side) = 12 units.

Thus, the perimeter of the triangle is 12 units, which corresponds to the option labeled B.

The calculation clearly shows how the perimeter is derived from the triangle's side lengths, leading to a straightforward understanding of perimeter calculation in geometry. The other options do not represent the correct sum of the sides, demonstrating how systematic addition is crucial in solving for the perimeter.