Final answer:
Root canal treatment and restoration is the preferred treatment with the most favorable long-term prognosis when the pulpal tissues of the mandibular first molar are exposed during caries removal.
Step-by-step explanation:
If the pulpal tissues of the mandibular first molar are exposed during caries removal, the preferred treatment that offers the most favorable long-term prognosis is Root canal treatment and restoration (Option A). Root canal treatment involves removing the infected pulp and filling the root canal with a biocompatible material. This helps to preserve the tooth and prevent further infection or abscess formation. Restoration is then done to restore the function and aesthetics of the tooth.