Chemistry: The Molecular Science (5th Edition)

"Base X" is the stronger base because its $K_b$ is larger.
When comparing the strength of bases, the one with a higher $K_b$ value is considered stronger. Analyzing the Kb values: $K_b$ (Base X) = $1 \times 10^{-4}$ and $K_b$ (Base Y) = $4 \times 10^{-5}$ Since $1 \times 10^{-4} > 4 \times 10^{-5}$, the "Base X" is stronger.