The correct answer is (c) T2* time
Explanation: When the gradient and RF coils are powered off, the magnetized elements get demagnetized and this causes the image to be formed. T1 is the time for the elements to go back to the original, demagnetized state. T2 is the time needed for the elements to go out of phase. T2* is the time needed for the spin to dephase. T2* is important for fMRI because oxygenated hemoglobin is diamagnetic while deoxygenated hemoglobin is paramagnetic. Thus, the T2* star time will be different for the two of them and the amount of oxygen used by the brain can be mapped.