On mannitol salt agar, growth surrounded by yellow halos indicates that the organism can sustain high salt concentrations and ferment mannitol. This color change occurs because some bacteria can ferment mannitol, producing acid as a byproduct, which lowers the pH of the agar and causes the phenol red indicator to turn yellow. Additionally, the ability to grow in high salt concentrations is a characteristic of organisms like Staphylococcus aureus, which ferments mannitol and forms yellow colonies with yellow halos on this specific agar medium.
So, the correct answer is:
c. the organism can sustain high salt concentrations and ferment mannitol
