Final answer:
The correct answer is option a. Thrasymachus is the interlocutor in Plato's 'The Republic' who claims that justice is the advantage of the stronger, challenging Socrates' view on the true nature of justice.
Step-by-step explanation:
The interlocutor in Plato's dialogues who asserts that justice is nothing but the advantage of the stronger is Thrasymachus. This assertion is made in the context of a debate about the nature of justice in Plato's work 'The Republic'.
Thrasymachus presents this view as a challenge to Socrates' inquiries about justice, suggesting that rulers make laws to benefit themselves, which is seen as just by those who must adhere to these laws. Thus, justice is nothing more than a conduit for the expression of power by the ruling class according to Thrasymachus.