Final answer:
The statement is true; a tropical storm is classified as a hurricane once it reaches sustained wind speeds of 74 mph, which is often when the distinctive eye forms.
Step-by-step explanation:
The statement "Because the distinctive eye forms at wind speeds of about 74 mph, this wind speed defines the threshold where a tropical storm has grown strong enough to be called a hurricane" is True.
Hurricanes develop from tropical depressions and are subsequently classified as tropical storms when their sustained wind speeds reach between 39 to 73 miles per hour. When these winds further increase to a sustained speed of 74 miles per hour, the storm is categorized as a hurricane, often distinguishable by the formation of its eye.