Step 1: Understanding the Concept:
The question asks for the name of a specific technology that uses a body of water to both collect and store solar energy as heat.
Step 2: Detailed Explanation:
Let's define the terms in the options:
(A) Solar cell (or Photovoltaic cell): This is a device that converts sunlight directly into electricity using the photovoltaic effect. It does not store energy as heat.
(B) Solar collector (or Solar thermal collector): This is a device designed to collect solar radiation and transfer the heat to a fluid (like water or air). While it collects heat, the primary long-term storage is usually done in a separate insulated tank, not within the collector itself.
(C) Solar still: This is a device that uses solar energy to evaporate water and then condense it to produce distilled (purified) water. Its primary purpose is water purification, not large-scale heat storage.
(D) Solar pond (or Salinity gradient solar pond): This is a large body of saline water (a pond) that is designed to collect and store solar energy as heat for a long term. It works by having a salinity gradient, with very salty water at the bottom and less salty water at the top. Sunlight penetrates to the dark-lined bottom and heats the water there. The high salt concentration at the bottom makes the water dense and prevents it from rising and losing heat through convection, which would happen in a normal pond. This allows the bottom layer to reach high temperatures (up to 90-100° C) and act as a massive thermal storage system. This description perfectly matches the question.
Step 3: Final Answer:
A body of water designed to collect and store solar heat is known as a solar pond. Therefore, option (D) is the correct answer.