Final answer:
The euphotic or photic zone of the ocean is where sunlight penetrates and supports photosynthesis, making it the habitat for phytoplankton and other photosynthetic organisms.
Step-by-step explanation:
The euphotic zone, also known as the photic zone, is the upper layer of ocean water in which enough sunlight penetrates for photosynthesis to occur. This zone supports a diverse range of photosynthetic organisms due to its access to sunlight. The type of organisms that live in the euphotic water zone and rely on photosynthesis are predominantly phytoplankton. These organisms are crucial to the marine ecosystem as they serve as the primary producers, forming the base of the food web. Other photosynthetic entities like algae also thrive in this zone. Therefore, the correct answer to the student's question is option (a) Phytoplankton.