Asynthesis reaction is a reaction between at least two compounds in which
a. one breaks down into at least two products.
b. a compound is decomposed by an electric current.
c. a compound burns in the presence of oxygen.
d. a new compound is formed.

d. a new compound is formed.

Explanation:

1. Decomposition is a type of chemical reaction in which one reactant gives two or more than two products.

Example:

2. Electrolytic decomposition is a type of chemical reaction in which one reactant gives two or more than two products. on passing electric current.

Example:

3. Combustion is a type of chemical reaction in which fuel is reacted with oxygen to form carbon dioxide and water.

Example:  

4. Synthesis reaction is a chemical reaction in which two reactants are combining to form one product.

Example:  

indicate valleys, hills, steepness and gentleness of slopes.

explanation:

contour lines on a map indicate elevation, or depth. numeric notations on contour lines refer to the level of elevation, or height about sea level.