A - Incorrect: Productive is an adjective, but it is being used to modify the verb operate, so it should be an adverb, productively.
C - Incorrect: Subject-verb agreement error: The relative pronoun that begins a new clause; the subject of the verb operates is managers, not study. The plural subject managers does not agree with the singular verb operates.
D - Incorrect: This answer choice changes the meaning of the sentence as the sentence is now about what makes managers productive and not what makes managers most productive.
E - Incorrect: The present progressive tense having is incorrect because the verb have is used to indicate a state or condition, so it cannot be in a progressive tense.
B - Correct: This answer choice correctly uses productively to modify the verb operate and uses the simple present tense have to indicate a condition or state.