Which of the following theories states that humans first migrated to america by boat?
the coastal crossing theory
the land bridge theory
the ice age theory
the asia theory

The theory that states that humans first migrated to America by boat is the the coastal crossing theory. This theory argues that there is a high probability that part of the settlement of America came from Europe, particularly from Iberia and 15,000 years ago, via the Atlantic Ocean.

The theory which talks about how humans first migrated to America by boat would be called the coastal crossing theory.

The other theories in question which are mentioned here all talk about different types of things, not necessarily related to humans migrating to America by boat.