The correct answer is (A) To look menacing (of clouds, sky, storm); to frown, scowl, glower, look sullen. Also, lower.

Lour comes from Middle English louren (to scowl), Dutch loeren, German lauern.

Examples of use:

• When he is annoyed, he lours at those persons who are near him.
• He walked purposefully as the dark clouds loured overhead.