Explanation

The correct option for the sentence "Leap years,--- have 366 days, contain an extra day in February." is D) "which" because it functions appropriately as a relative pronoun that introduces a non-defining relative clause.

In this context, "which" helps to provide additional information about "leap years" without changing the fundamental meaning of the sentence. The phrase "which have 366 days, contain an extra day in February" clarifies what leap years are, situating them within a specific context of their unique characteristics.

The other options do not fit as well. A) "that" would typically be used in restrictive clauses, which are essential to the meaning of the sentence; however, in this case, the clause adds detail rather than restricting meaning. B) "those" cannot function in this structure as it would require a different syntactic arrangement, and C) "what" does not correctly relate to the noun "leap years." Thus, "which" is the most grammatically suitable choice, maintaining the flow and clarity of the sentence.