There are scientific ways at which man solves problems.

A
in which指代何种方法
男人解决问题有很多种科学方法

A
这是非限定性定语从句,做时你可以变换下顺序,
为man solves problems____scientific ways.
很容易选出来用in,in scientific ways:用科学的方法.

with which
by which 都可以
说的就是人用科学的方法解决问题

选A
介词in+which结构

选A。in a way用···方法

its A, cause thats the one which makes most sence