Which of the following is a clear indicator that the tire is at the end of its legal life? Select one:
A. Out-of-Balance Conditions
B. Tread depth above the level of built-in wear indicators
C. Low tire pressure
D. Tread depth below the level of built-in wear indicators

Option D, Tread depth below the level of built-in wear indicators

Explanation:

The legal life of tire is checked by  looking out for tread wear indicators. In general, tread wear indicators are nothing but small bars of width of 5 mm. These bars are built in the main grooves of the tires.

If the tire tread is at the top level of the tread wear indicating bar, then it means that tires have reached the legal limit of tire death (1.6 mm)

Hence, option D is correct