How can oil reach the valve train?

Oil can reach the valve train via oil galleries and hollow push rods, which are integral components of an engine's lubrication system. Oil galleries are pathways within the engine that allow oil to flow from the oil pump to various components, including the valve train. These galleries ensure that the oil is distributed evenly and efficiently throughout the engine.

Hollow push rods play a crucial role in this process as they are designed to not only transfer motion from the camshaft to the rocker arms but also to carry oil to the upper parts of the engine. By utilizing hollow push rods, the engine can provide additional lubrication to areas that require it, thereby reducing wear and promoting longevity in the valve train components.

The other choices suggest alternate methods of oil delivery that do not encompass the complete system or the common practices in modern engine designs. For instance, relying solely on external lines or direct injection does not accurately reflect how most engines are designed to lubricate the valve train, nor does concentrating solely on the oil sump capture the complexity and efficiency of modern lubrication strategies.