A

In magnetism, the relation given by Pythagoras is often used for calculating vector quantities such as current, voltage, and their respective components.
Step 1: The first equation states that the square of the total current $I^2$ is equal to the sum of the squares of its components: $I^2 = V^2 + H^2$. This indicates a right triangle formed by the current vector, where $V$ and $H$ are the other two sides.
Step 2: The other options present incorrect or misleading relationships that don't adhere to the principles of magnetism or vector analysis. Hence, the first relation is the only correct representation.
Therefore, A.