答案：A some of which measure 50 meters.
翻译：这森林里有如此多的高数,有一些高达50米.
句中用逗号隔开,其后又没有并列连词and等连接,说明后半部分为非限制性定语从句.
关系代词代指先行词trees,在定语从句中充当of的宾语.
其中,measures为定语从句的谓语动词,意思是“度量为...”,所以不能选用答案B,因为measuring不能单独做句子的谓语.
即：some of (which) measure 50 metres
= some of (the tall trees) measure 50 metres
注：是不是楼主打错了答案A,其中的谓语动词meausre不能使用单三形式,因为该定语从句的主语some of which表复数概念.