Final answer:
A nurse should instruct the client to immerse her perineum in warm water for 20 minutes as a sitz bath postpartum for a first-degree tear, as this will help reduce inflammation and promote healing.
Step-by-step explanation:
The correct intervention by the nurse for a client who is 28 hours postpartum with a first-degree tear and swollen perineum requesting assistance with her first sitz bath is option a) Instruct the client to immerse her perineum in warm water for 20 minutes. This helps to relieve discomfort, reduce inflammation, and promote healing of the perineal area. Adding bath salts (option b) is not typically recommended because it can cause irritation to sensitive tissues, while the use of ice packs (option c) is more commonly advised immediately postpartum, and not just before taking a sitz bath. Analgesic creams (option d) can be used, but this should be under the guidance of a healthcare provider to prevent irritation to the healing tissues.