The kind of molecule that provides building blocks for tissues, transports other molecules, and helps regulate certain reactions in the body is a protein.

Here's why:

* Building Blocks for Tissues: Proteins are the primary structural components of tissues. They form the framework of muscles, bones, skin, and other tissues. Collagen and keratin are examples of proteins that provide structure.

* Transporting Other Molecules: Proteins act as carriers for many molecules within the body. For example, hemoglobin in red blood cells transports oxygen, and lipoproteins transport fats.

* Regulating Reactions: Enzymes, which are proteins, catalyze (speed up) biochemical reactions in the body. They play a crucial role in metabolism, digestion, and many other processes.

While other molecules like carbohydrates and lipids also contribute to the body's functions, proteins are unique in their versatility and crucial roles in these three major areas.