The correct option is should.
It should be our concerted effort not to employ children in industries and to save them from any kind of exploitation.
"Should" expresses moral obligation or duty, which fits the concluding tone of an article appealing to collective responsibility. "May" suggests possibility, and "could" suggests ability — neither conveys the strong moral appeal needed here. Examiners expect students to recognise modal verbs that suit the context: a conclusion calling for action requires "should."