A constituent of diesel oil that collects in the DPF is ____________.

The correct answer, which is ash, relates to the diesel particulate filter (DPF) and its function. During the combustion of diesel fuel, certain components do not combust completely and can lead to particulate matter being generated. This particulate matter includes unburned fuel, soot, and other byproducts of combustion.

In the context of a DPF, ash refers to inorganic materials and additives present in diesel fuel, as well as materials that might derive from engine oil and oil additives. Over time, this ash accumulates in the DPF, as it is not burned off during the regeneration process that primarily targets carbon-based soot.

While sludge, carbon, and silica could have roles in engine operations or conditions, they do not specifically represent the constituents that are collected and remain in the DPF as thoroughly as ash does. Carbon is a primary particulate that gets trapped in the DPF temporarily until it is removed during regeneration, but ash is what consistently builds up and necessitates eventual maintenance of the filter. This distinction clarifies why ash is the appropriate term associated with accumulation within a DPF.