The correct answer is B) An electrically driven auxiliary blower is usually provided because the turbocharger cannot provide enough air at low engine speeds and loads. The reasoning behind this is that at low engine speeds and loads, the turbocharger may not be able to provide enough air to the engine due to insufficient exhaust gas flow to drive the turbocharger. In such cases, an electrically driven auxiliary blower is typically used to supplement the turbocharger and ensure adequate air supply to the engine, especially during low-load conditions. The other options are incorrect because: A) A mechanically driven auxiliary blower is not commonly used in modern turbocharged slow-speed two-stroke cycle diesel engines, as they are less efficient and less flexible than electrically driven blowers. C) An electrically driven auxiliary blower is used to provide more air at low engine speeds and loads, not high speeds and loads. D) A mechanically driven auxiliary blower is not usually provided, as it is less effective than an electrically driven blower in addressing the air supply issues at low engine speeds and loads.