选b,many of which
which指代的上文中snakes,他们的特点是极其危险.这是个非限定性定语从句.
不能用many of them.如果要这样用,many前必须加and,或者many前是句号,many引导的是一个完整的句子.否则整个句子就不完整了.
正确用法：
in word ,there are over 2000 varieties of snakes,many of which are extremely dangerous.
in word ,there are over 2000 varieties of snakes,and many of them are extremely dangerous.
in word ,there are over 2000 varieties of snakes.Many of them are extremely dangerous.