Which process requires an external power source? (1) neutralization (3) fermentation(2) synthesis (4) electrolysis

The correct answer is option (4).

Explanation:

Electrolysis it is chemical technique used to separate out the constituent of solution by passing an electricity through it. Electricity is supplied to the solution is done by the external power sources.

For example:

Neutralization is the type of chemical reaction which an acid reacts with base along with the formation of water molecule and salt.

Fermentation is chemical reaction in which substance is break down into simpler  compounds by the action of microorganism.Generally sugar or carbohydrates are broken down into alcohols or acids.

Synthesis  is chemical reaction which two or more simpler compounds fused to together to give a single new compound.