Final answer:
The client who chooses to formula-feed should wear a supportive brassiere continuously for about 72 hours to help with breast engorgement and discourage milk production.
Step-by-step explanation:
The correct instruction for a client who has chosen to formula-feed her infant and is not breastfeeding is to wear a supportive brassiere continuously for about 72 hours. This practice helps to support the breasts during the period of engorgement and can aid in the suppression of lactation. Neither applying a heating pad, using breast shields for inversion, nor using a manual breast pump is recommended in this situation since the goal is to discourage milk production and manage discomfort associated with engorgement as lactation ceases.