The document discusses the criteria for various sets to qualify as subspaces of the polynomial space p_2. It analyzes three sets: one is proven not to be a subspace, while another set formed by the span of {1+x, 1+x^2} is confirmed to be a subspace. The document concludes that the set formed by {1, x, x^2, x^3} does not constitute a subspace as p_2 is not contained within it.