The correct option is (c) MUST
"MUST we cut trees and destroy natural habitat? Stop! Before it is too late."
Must is used here to express a rhetorical question conveying strong moral obligation/criticism.
"Must" is used to express strong obligation or moral compulsion. In rhetorical slogans, "must we…?" conveys a strong reproach — questioning whether we are compelled to do something harmful. The other options (may, might, could) suggest permission or possibility, which weakens the impact of the slogan. Examiners expect students to identify the modal that best fits the tone of urgency and moral responsibility in a public awareness slogan.