Final answer:
The detail mentioning the male narwhal's long, protruding tusk best supports the claim that narwhals are known for their tusks.
Step-by-step explanation:
The detail that best supports the author's claim that the narwhal is best known for its tusk is: "Instead, male narwhals have a single, long, straight tooth (or tusk) that protrudes two to three meters out of the upper left jaw." This unique physical characteristic is the most prominent and distinctive feature of the narwhal, making it the reason for their fame and the association with legendary unicorns.