What is an invitation to treat? Which of the following is correct?

Select one:

a.Invitations to treat are a form of subtle offers, once an invitation to treat is accepted, a contract is formed

b.An invitation to treat is a direct offer to a member of the public, which can take place by the placement of goods on a supermarket shelf

c.An invitation to treat is a statement, display of goods or advertising which invites a potential contractor to make an offer

d.While advertisements are invitations to treat, words inducing another person to enter a contract cannot be considered to be invitations but are actual offers