Final answer:
Sentence B is correctly punctuated because the essential subordinate clause 'that you spend on useless things' is integral to the sentence and is not separated by commas.
Step-by-step explanation:
When determining the correct punctuation of subordinate clauses, it's important to recognize whether the information within the clause is essential or nonessential. An essential subordinate clause is integral to the sentence's meaning and should not be separated by commas. Conversely, nonessential clauses, which provide additional information that can be omitted without changing the basic meaning of the sentence, should be set off with commas.
In the given options:
- 'The thing, that is amazing, is photographing a black hole using the world as a telescope.' is incorrect because the clause 'that is amazing' is essential to the meaning of the sentence and therefore should not have commas.
- 'The time that you spend on useless things is time you can never reclaim.' is correct because the clause 'that you spend on useless things' is essential and is punctuated correctly without commas.
- 'The subjects, that are the most difficult, are usually the most valuable.' is incorrect because the clause 'that are the most difficult' is essential and should not be surrounded by commas.
Therefore, Sentence B correctly punctuates the essential subordinate clause.