Building blocks of rocks:
Rocks are composed of various substances and materials that are classified into several building blocks. These building blocks are the fundamental components that make up rocks. The correct answer to this question is option D, minerals. Let's discuss each option in detail to understand why minerals are considered the building blocks of rocks.

Sand:
Sand is a granular material composed of small rock and mineral particles. It is primarily made up of fragments of minerals, rocks, and shells. While sand is a component of rocks, it is not considered a building block itself because it is made up of a mixture of various materials rather than a single substance.

Pebbles:
Pebbles are small, rounded rocks that are larger than sand but smaller than stones. They are often found along riverbeds and shorelines. Pebbles are made up of different types of rocks, minerals, and sometimes fossils. Similar to sand, pebbles are not considered building blocks on their own as they are composed of a mixture of materials.

Stone:
Stone is a general term used to describe any solid, naturally occurring substance composed of minerals. It refers to rocks that are large and hard, usually used for construction purposes. While stones can be considered building blocks, they are not as specific as minerals, which are the individual components that make up rocks.

Minerals:
Minerals are the building blocks of rocks. They are naturally occurring, inorganic substances that have a specific chemical composition and a crystal structure. Minerals can be found in various types of rocks and are the primary components that give rocks their unique characteristics. Different minerals have different properties, such as color, hardness, and luster, which contribute to the overall appearance and composition of rocks.

In conclusion, minerals are the correct answer as they are the fundamental components that make up rocks. While sand, pebbles, and stones are all associated with rocks, they are not considered building blocks themselves as they are composed of mixtures or larger aggregates of minerals and rocks.