Which of the following mistakes will NOT render a contract invalid?

a. The buyer has mistakenly purchased the wrong colour product.

b. The buyer has purchased a product that they mistakenly believe to be a valuable painting

and the seller knows the buyer's mistaken belief.

c. The buyer has agreed to purchase shares in a company and unknown to both the seller

and buyer the company has gone bankrupt.

d. The buyer thought that they had agreed to purchase product "x" from the seller, and the

seller thought that they had agreed to sell product "y".