Which phrases and clauses are characterized by not being separated from the rest of the sentence by commas?

The correct choice focuses on restrictive phrases and clauses, which play an essential role in providing necessary information about a noun in a sentence. Unlike nonrestrictive phrases and clauses, which set off additional, non-essential information and are typically separated by commas, restrictive elements are integral to the meaning of the sentence. They specify or define the noun being referred to without which the sentence's message would become unclear or less precise.

For instance, in the sentence "The book that has the blue cover is mine," the clause "that has the blue cover" is restrictive because it identifies which specific book is meant. If we removed this clause, the meaning would change, indicating that the speaker could be referring to any book.

Other types of clauses and phrases, like nonrestrictive, conditional, and dependent clauses, either add supplementary information, imply conditions or scenarios, or rely on the main clause to form a complete thought. These characteristics influence their punctuation and the way they impact the overall sentence structure.