This is the town---------he wants to visit so much.

which
This is the town which he wants to visit so much.
因为visit 后面直接加的是the town
visit the town
visit是及物动词

填which
你看哦，把先行词填到后面去，就是：HE WANTS TO VISIT THE TOWN SO MUCH 是通顺的，不需要加介词，就用WHICH，如果要用介词，用WHERE．

填which/that，分句中缺少宾语，所以是the town which he wants to visit,which作为关系代词填入句中充当成分

which
如果要用what，前边的the town就没用了

填which或that
定语从句，先行词是town，从句中缺少宾语。