Correct Option
The correct option isRocks formed when molten magma cools and becomes solid.
Explanation
Primary Rocks, scientifically known as Igneous Rocks, are the fundamental rocks of the Earth's crust. They are termed "primary" because they are the first to form and constitute the parent material from which other rock types (sedimentary and metamorphic) are derived.
Option Analysis
- Rocks formed by the deposition of sediments. is Incorrect: Rocks formed by the accumulation, deposition, and lithification of sediments are known as Sedimentary Rocks (e.g., Sandstone, Shale).
- Rocks formed when molten magma cools and becomes solid. is Correct: Igneous Rocks are formed when molten magma (from the mantle) or lava (on the surface) cools and solidifies. Because they form directly from the molten state without being derived from pre-existing rocks, they are called Primary Rocks. Examples include Granite and Basalt.
- Rocks changed under great heat and pressure. is Incorrect: Rocks that change their form and composition under the influence of intense heat and pressure are known as Metamorphic Rocks (e.g., Marble, Slate).
- Rocks that contain fossils of plants and animals. is Incorrect: Fossils are characteristic of Sedimentary Rocks. Primary rocks form at extremely high temperatures that destroy organic matter, rendering them unfossiliferous.
Key Takeaway: Primary Rocks (Igneous) originate directly from the solidification of magma or lava and are devoid of fossils.