Reaction
a chemical reaction that has the general formula of ab - a + b is best classified as a
synthesis
polymerization
decomposition
oxidation
replacement

Decomposition.

Explanation:

Decomposition reaction:

It is the reaction in which one reactant is break down into two or more product.

AB → A + B

While other options are incorrect because,

A) Synthesis:

Synthesis reaction:

It is the reaction in which two or more simple substance react to give one or more complex product.

A+B → AB

Polymerization:

It is the reaction in which monomers are combine to form long chain polymer.

Oxidation:

Oxidation involve the removal of electrons and oxidation state of atom of an element is increased.

Replacement:

Single replacement:

It is the reaction in which one elements replace the other element in compound.

AB + C → AC + B

Double replacement:

It is the reaction in which two compound exchange their ions and form new compounds.

AB + CD → AC +BD

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.