Question

Spartan soldiers helped save retreating members of the Greek army at the Battle of Marathon. at the Battle of Thermopylae. against a force of three hundred Persian troops. against the strength of the Athenian navy.

Answer 1

Spartan soldiers helped save retreating members of the Greek army

   B: at the Battle of Thermopylae.

Answer 2

I'm somewhat confused at this question, but will give my best to answer it. 

Spartan soldiers helped save retreating members of the Greek army:

against a force of three hundred Persian troops - this surely is false, as there never was a three hundred strong Persian force.

against the strength of the Athenian navy - this doesn't make much sense either, as the Athenian navy was Greek, so there's no reason for Spartan soldiers to defend greeks against the Athenian navy.

at the Battle of Marathon - the Spartans were not involved in this battle as they were celebrating a religious festival at the time.

The most likely answer is therefore at the Battle of Thermopylae, where a small Spartan contingent of soldiers stayed to defend the frontiers of their borders in order to enable the rest of the army that was with them at the time, to retreat.