Scientists have created the world's smallest "snowman",(      )about a fifth of the width of a human hair.

A measuring   B measures   C measured   D to measure 

答：最佳选项是A。

measuring  about a fifth of the width of a human hair 是现在分词短语，是非限制性定语

相当于 非限制性定语从句 which measures  about a fifth of the width of a human hair. 

其逻辑主语是snowman, measure在这里和weigh的用法是一样的，表示“量起来有。。。”

不用被动语态。