A puff back is an ____________ caused by an accumulation of the air-oil mixture in the combustion chamber.

A puff back is an explosion that occurs as a result of an accumulation of the air-oil mixture in the combustion chamber of a heating system. When the mixture is ignited, the sudden release of energy causes a backflow of flames and combustion gases, leading to a puffing back effect. This phenomenon can occur when there's incomplete combustion or a malfunction in the burner, causing the fuel to accumulate and then ignite explosively.

The other options do not accurately characterize the event. Ignition failure refers to a situation where the burner fails to ignite, which is different from the explosive nature of a puff back. A smoke event typically describes the release of smoke due to improper combustion but does not capture the explosive characteristic. A fuel leak involves escaping fuel without combustion, which again does not relate to the explosive reaction of a puff back. Therefore, defining a puff back as an explosion best captures the risk and mechanics involved in this scenario.