W is close to Vancouver which is in a predominantly coniferous biome.

Y is in the transition zone between the tropical rainforest at the equator and the Sahara desert in northern Africa. This transition zone is the savanna biome. Notice on the map that significant lines of latitude have not been drawn in. One should remember the location of the equator as well as 30 degrees north and 30 degrees south.

Z is in the desert biome of Saudi Arabia.

Therefore, the answer is X (B) - the dominant deciduous forest of the northeast United States.