Answer:
third
Step-by-step explanation:
The utility for consuming the first donut is 9 utils.
The utility for consuming the second donut is the difference from the total utility for the first two and just the first donut.

The utility for consuming the third donut is the difference from the total utility for the first three and just the first two donuts.

The utility for consuming the fourth donut is the difference from the total utility for the first four and just the first three donuts.

Since the utility for the 4th donut is less than the utility for the 3rd donut, utility begins to diminish after consuming the third donut.