Final answer:
Implied by the statement, "A side hustle you can do while you're still at your 9-5" in the context of career development is the potential for additional income sources.
Step-by-step explanation:
Implied by the statement, "A side hustle you can do while you're still at your 9-5" in the context of career development is d) The potential for additional income sources. A side hustle refers to an extra job or project that someone takes on alongside their full-time job. It provides an opportunity to earn extra money and diversify income sources, which can be beneficial for career development.