The correct answer is C) during initial starting. During the initial starting of a diesel engine, the lubrication system is not yet fully primed and the engine components are not yet at their optimal operating temperature. This makes the initial starting the most crucial time for proper lubrication, as the engine parts are vulnerable to wear and damage without adequate oil flow and viscosity. The other options are incorrect because: A) low load periods do not pose the same risk as the initial startup, B) the first warm-up period is important but not as critical as the initial start, and D) load reversals, while requiring attention, are not as vital as the initial startup for diesel engine lubrication.