Which word does NOT belong with the others?

Lets check all option,

Option (a):

Bay: A bay is a body of water that is partially surrounded by land, typically with an opening to the sea.

Option (b):
Peninsula: A peninsula is a piece of land that is surrounded by water on three sides but remains connected to the mainland.

Option (c):
Island: An island is a piece of land completely surrounded by water.

Option (d):
Cape: A cape is a point of land that extends into a body of water, typically a sea or ocean.​​

Now, option (c), is odd in all options because Island does not belong with the others because it is the only one that is completely surrounded by water, while the others (bay, peninsula, cape) are all landforms that involve connections to the mainland.

Thus, the correct answer is (c).