Emulsion is a type of colloidal dispersion in which two immiscible liquids are mixed together to form a stable mixture. The emulsion is composed of small droplets of one liquid dispersed in another liquid. In this case, the correct pair of liquids that form an emulsion is Water and coconut oil.

Explanation:

1. H2O and C2H5OH:
- Water (H2O) and ethyl alcohol (C2H5OH) are both polar liquids.
- They can mix together to form a homogeneous solution, but they do not form an emulsion.
- Since both liquids are polar, they are miscible and do not separate into distinct phases.

2. C2H5OH and C2H5OC2H5:
- Both ethyl alcohol (C2H5OH) and diethyl ether (C2H5OC2H5) are organic solvents.
- They are miscible with each other and can form a homogeneous solution.
- However, they do not form an emulsion because they are not immiscible liquids.

3. Water and coconut oil:
- Water (H2O) is a polar liquid, while coconut oil is a nonpolar liquid.
- Polar and nonpolar liquids are immiscible and do not mix together easily.
- However, with the help of an emulsifying agent, such as a surfactant or an emulsifier, water and coconut oil can form an emulsion.
- Emulsifying agents have both hydrophobic (water-repelling) and hydrophilic (water-attracting) properties.
- These agents surround and stabilize the oil droplets in water by forming a protective layer around them.
- This prevents the oil droplets from coalescing and keeps the emulsion stable.

4. Coconut oil and NaOH:
- Coconut oil is a nonpolar liquid, while sodium hydroxide (NaOH) is a polar compound.
- Nonpolar and polar substances do not mix together easily and typically form separate layers.
- Therefore, coconut oil and sodium hydroxide do not form an emulsion.

In conclusion, the pair of liquids that form an emulsion is Water and coconut oil.