O, an Eccentric Millionaire, Offers a Prize of £500,000 to Anyone

O, an eccentric millionaire, offers a prize of £500,000 to anyone who can travel to the Himalayas and bring a plant sample back to her within two months. A takes up the challenge and invests a large sum into preparation for the journey. A week after setting off, and virtually certain to succeed in the task, A learns that O has revoked the offer. Does A have a remedy, and if so, what is the most likely remedy available?

A) Yes, A can sue O in an action for breach of contract for £500,000.
B) Yes, A can sue O for damages for breach of contract for the expense she has incurred.
C) Yes, she can still claim £500,000 if she completes the journey and returns with the sample before the stipulated deadline.
D) No, since she has not performed the requested act.