”Reasoning” is singular, and the correct form of the verb to match this subject is ”provide.”
The correct article is ”the” because we are referring to a specific concept (”a reliable basis for a theory of justice”) which has already been introduced in the sentence.
The word ”concerned” is the correct form to indicate the issue is about something being debated or considered.
Read the sentence and infer the writer's tone: "The politician's speech was filled with lofty promises and little substance, a performance repeated every election season."