这个句子为强调句型，还原为普通句式为：

The demand of individual consumers, coupled with the desire of businessmen to maximize profits and the desire of individuals to maximize their incomes, together determine what shall be produced and how resources are used to produce it.

再简化一下句子：

The demand of A, couple with desire of B and the desire of C, determine D. 

从语法上讲，句子的主语为单数名词the demand, 谓语动词应该用单数determines。从语义上讲，决定D的，是A的demand，B和C的desire，所以按照语义原则，谓语用了复数形式。这就是主谓一致中讲到的所谓语法一致和语义一致原则。前者为正式语体，后者为非正式语体。考试要根据语法一致原则用单数谓语。实际上，所谓语义一致原则是犯错误的人太多，而被认可了。