Mobile phones area boon for people.

The mobile phoneis a boon for people.

A mobile phone isa boon for people.

请问老师三句话是不是都正不正确?怎么样理解冠词和前面名词复数之间的关系?

可以参考“英语语法网”的下面这篇文章(第1条):

http://www.yygrammar.com/Article/201908/5484.html