To answer this question, the user needs to have knowledge of the different types of datasets in mainframe.

The default cluster type in mainframe is KSDS (Key Sequenced Dataset).

Option A) ESDS (Entry Sequenced Dataset) is incorrect because it is not the default cluster type.

Option B) RRDS (Relative Record Dataset) is incorrect because it is not the default cluster type.

Option C) LDS (Linear Dataset) is incorrect because it is not a type of clustered dataset.

Option D) KSDS (Key Sequenced Dataset) is the correct answer. It is the default cluster type used in mainframes.

Therefore, the answer is: D. KSDS.