Which one of the following would not be a valid contract made under seal?

A gratuitous written promise made under seal to donate $$400$ to the Red Cross.

A contract made under seal after commencing employment where an employee promised never to reveal the employer$$s trade secrets to any competitor.

A contract made under seal by a $17-$year$-$old to buy a big screen TV$,$ with the agreement to pay $$1,200,$ the fair market price.

A written promise made under seal to paint a house for $$400.$

A signed formal deed under seal to transfer a house to a favourite nephew.