Which of the following is most likely to mark the boundary between two continents?
a. river.
b. group of hills.
c. a lake.
d. an ocean.

Of the options provided, that which most likely marks the boundary between two continents is

Oceans are large bodies of water that take up to 70% of the earth's surface. Continents are made up of countries. Due to their size, oceans will most likely be boundaries for continents.

97% of the water in the earth comes from oceans. The Arctic ocean separates the European and Asian continents.

The North Pacific Ocean separates North America from Asia, and the Atlantic Ocean separates Europe from North America.

Summarily, an ocean is most likely to mark the boundary between two continents.

Learn more about oceans here:

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c. the king has significantly abused the colonists' rights many times over a long period of time.