Final answer:
After a red giant star begins to burn helium in what is known as the helium flash, its diameter decreases due to the contraction of the core and the temporary increase in surface temperature.
Step-by-step explanation:
When a red giant star begins to burn helium, there is a process known as the 'helium flash,' which leads to a readjustment of the star's internal structure. During this phase, the star's core contracts while its outer layers expand. After the helium flash and the onset of stable helium burning, the star shrinks somewhat as it moves to a higher temperature and lower luminosity, which implies that the diameter of the star decreases. This stage is characterized by the fusion of helium into carbon and sometimes oxygen, marking a brief period of stability.
As the star continues to burn helium in its core, it maintains an equilibrium similar to the main-sequence stage. However, once the helium is exhausted, the star will again become a red giant, increasing in size before eventually nearing the end of its life. Therefore, after the helium flash, but before the helium is exhausted, the correct answer is that the diameter of the red giant star decreases.