Language is a main means of c_______.这里的communication 什么时候communication加s呢?

是 communication
communication 作“交流”,“联系”讲时 是抽象名词, 一般不加s

a means/method/system of communication
交流手段/ 方式/ 系统:
再如：
Letters are their only means of communication.
书信是他们唯一的联系方式.

作“ 通信工具” 讲时是可数名词
如：
All communications with the north have been stopped by snowstorm.
北部的一切交通均为暴风雪所阻.