Which one is pure substance?

1) The boiling point of pure water at sea level is $100^\circ\text{C}$.
The correct option is a).

$$\boxed{\text{a) } 100^\circ\text{C}}$$

2) The freezing point of pure water at sea level is $0^\circ\text{C}$.
The correct option is b).

$$\boxed{\text{b) } 0^\circ\text{C}}$$

3) A pure substance has a definite and uniform composition.

• Common salt (sodium chloride, NaCl) is a compound, which is a type of pure substance.
• Salt solution and sugar solution are homogeneous mixtures.
• Brass is an alloy, which is a homogeneous mixture. The correct option is a).

$$\boxed{\text{a) common salt}}$$

4) A homogeneous mixture has a uniform composition throughout, and its components are indistinguishable.

• Alcohol and water mix completely to form a uniform solution.
• Sand and water, muddy water, and sand and sugar are all heterogeneous mixtures, where components can be distinguished. The correct option is a).

$$\boxed{\text{a) alcohol and water}}$$

5) A compound is a pure substance formed from two or more elements chemically bonded together.

• Common salt (NaCl) is a compound.
• Sugar (e.g., sucrose, C$_{12}$H$_{22}$O$_{11}$) is a compound.
• Muddy water is a heterogeneous mixture of water and soil particles. It is not a compound.
• Air is a homogeneous mixture of gases (primarily nitrogen, oxygen, argon, carbon dioxide, etc.). It is also not a compound. Given the options, both muddy water and air are mixtures and therefore not compounds. Muddy water is a clear example of a mixture. The correct option is c).

$$\boxed{\text{c) muddy water}}$$