The correct options are:
- registered dietitian (RD).
- registered dietitian nutritionist (RDN).
The titles "registered dietitian" and "registered dietitian nutritionist" refer to professionals who are credentialed and have met specific educational and training requirements in nutrition and dietetics. The other titles listed, such as "certified fitness and nutrition trainer" and "nutritionist," do not necessarily imply the same level of formal education and credentialing in nutrition.