When lead nitrate is heated, it breaks down into lead monoxide, nitrogen dioxide and oxygen.
 

$2\mathit{Pb}{(\mathit{NO}}_3{)}_2\to 2\mathit{PbO}+4N{O}_2+O_2$

This reaction is an example of:

In a combination reaction, two or more reactants combine to form a single product. Thus, the given reaction is not a combination reaction.

In a decomposition reaction, one reactant is decomposed (broken down) into two or more two products. This is what is happening in the given reaction, therefore, it is a decomposition reaction.

In a displacement reaction, one element of the compound is replaced by a similar type of element.

If a compound is complex in nature, it breaks down into simpler compounds, it is called a decomposition reaction.
As seen in the reaction, the lead nitrate ( ${\text{Pb(NO}}_{\text{3}}{\text{)}}_{\text{2}}$) is breaking down to form lead oxide, nitrogen dioxide, and oxygen.

Thus, the given reaction is a decomposition reaction.