In the following truth table, $V = 1$ if and only if the input is valid. What function does the truth table represent?

Analyzing the truth table:

• The input bits are $D_0, D_1, D_2, D_3$, with $D_3$ having the highest priority down to $D_0$.

• When $D_3 = 1$, regardless of lower inputs ($x x x 1$), the output is $X_1 X_0 = 11$.

• When $D_3 = 0$ and $D_2 = 1$ ($x x 1 0$), the output is $X_1 X_0 = 10$.

• When $D_3 = 0, D_2 = 0$ and $D_1 = 1$ ($x 1 0 0$), the output is $X_1 X_0 = 01$.

• When $D_3 = 0, D_2 = 0, D_1 = 0$ and $D_0 = 1$ ($1 0 0 0$), the output is $X_1 X_0 = 00$.

• When all inputs are 0 ($0 0 0 0$), $V = 0$, indicating an invalid input.

This behavior matches a 4-to-2 priority encoder, where higher index inputs take precedence over lower index inputs, and a valid bit $V$ is asserted when at least one input is active.

Therefore, the correct option is A.