Final answer:
The conjunction 'A menos que' always requires the use of the subjunctive mood because it introduces a conditional statement that is not guaranteed or is contrary to fact.
Step-by-step explanation:
The conjunction that always requires the subjunctive mood when used in a sentence is A) "A menos que".
When "a menos que" is used, it introduces a condition that is contrary to fact or not yet realized, which is why the subjunctive is necessary.
For instance, the sentence 'No iré al parque a menos que haga buen tiempo' translates to 'I will not go to the park unless the weather is good,' showing a condition that is not currently met and hence requires the subjunctive.