From what you know about higgins, what is the most likely reason he becomes angry when she attempts to return the ring he bought her?

he is aghast that she would defy the rules of good manners and decorum.

he is ashamed that she would reject a good profit.

he is frightened that she is slipping out of his control.

he is angered that she would reject his wedding proposal.

(C) He is frightened that she is slipping out of his control

Explanation:

When Eliza does this, she is becoming more independent even though she is facing Higgins' bullying superiority. Higgins sometimes treats people like objects, as shows in another occasion when Eliza flings his slippers in his face, which shows that she means no more to him than his slippers.  

Not only does Eliza is the object to being treated like an object, but she also goes on to assert herself by saying that she would never sell herself as Higgins suggests. This makes Higgins' perspective change, on what a woman can be, and, as shown in the final act of Pygmalion, finally makes him realize that he should treat Eliza as someone equal to him rather than a burden.