The rule is that if the language of the offer and the surrounding circumstances do not indicate what manner of acceptance is invited, then the offer invites acceptance in any way and by any medium that is reasonable in the circumstances.
To accept Richard and Frances must (1) a manifestation of a willingness to enter the bargain proposed by the offer (2) in a manner invited or required by the offeror. Since the offer invites acceptance in any way and by any medium that is reasonable in the circumstances, Richard and Frances must in a way and by a medium that is reasonable in the circumstances.
Whether this is by promising or by performing depends on the circumstances. The following question and answer sequence leads to the conclusion that, in Klockner v. Green it would have been reasonable in the circumstances to accept by performing.