In “the lady of shalott” by alfred, lord tennyson, what causes the curse to take effect? a. the mirror cracks from one side to the other. b. the lady of shalott looks directly down at camelot. c. the lady of shalott stops weaving. d. sir lancelot rides by on his horse.

D. Sir Lancelot rides by on his horse. 

Lady Shalott was a lady living all alone in a tower near Camelot. She's cursed and she didn't know what the curse was. She must only weave images on her loom reflected from her mirror the busy life of the people in Camelot. But she didn't look directly down there. 
Until one day, she saw Sir Lancelot riding by on his horse down to Camelot. She was fascinated by the knight's face, so she stopped weaving and looked directly down at Camelot to where he was heading. Because of this incident, the mirror cracked from one side to the other, bringing about the curse. 

The main cause is the appearance of Sir Lancelot to the lady's eyes because that causes her to stop weaving and to look directly down Camelot which therefore causes the mirror to crack. The cracking of the mirror is only a sign that the curse has taken effect not the cause.