Answer :
(a)

This reaction is a synthesis reaction.
(b)

This reaction is a double-displacement reaction.
(c)

This reaction is a decomposition reaction.
(d)

This reaction is a single displacement reaction.
Explanation :
Synthesis reaction : A chemical reaction where multiple substances or reactants combine to form a single product.
It is represented as,

Combustion reaction : A chemical reaction in which a hydrocarbon reaction with the oxygen to give product as carbon dioxide and water.
It is represented as,

Single replacement reaction : A chemical reaction in which the more reactive element replace the less reactive element.
It is represented as,

In this reaction, A is more reactive element and B is less reactive element.
Double replacement reaction : It is a type of chemical reaction where a positive cation and a negative anion of two reactants exchange places to form two new products.
It is represented as,

(X and A are the cations, Y and B are the anions)
Decomposition reaction : A chemical reaction where single larger substance or reactant decompose to give two or more products.
It is represented as,

(a)

This reaction is a synthesis reaction.
(b)

This reaction is a double-displacement reaction.
(c)

This reaction is a decomposition reaction.
(d)

This reaction is a single displacement reaction.