If the visiting card is used "R.S.V.P." is necessary, because usually invitations on visiting cards do not presuppose answers. The reply to the above may be either formal, in the third person, or may be an informal note.
(B)
500 Park Avenue,
January 4, 1920.
My dear Mrs. Elliott,
Will you and Mr. Elliott give us the pleasure of your company on Thursday, January the eighteenth, at ten o'clock? We are planning an informal dance and we should be so glad to have you with us.
Cordially yours,
Katherine G. Evans.
An acknowledgment should be sent within a week. Never acknowledge a visiting-card invitation by a visiting card. An informal note of acceptance or regret is proper.
Accepting