What agarose concentration is generally helpful for separating large DNA fragments?

Correct answer: lower concentration / Lower concentration

Agarose gel contains a network of pores through which DNA fragments move under an electric field. Lower agarose concentration produces a looser gel with larger pores. Large DNA fragments can pass through such a network more easily, so their migration and separation are generally improved. Higher agarose concentrations make smaller pores and are more useful for resolving relatively small fragments.

Therefore option B is correct for large fragments. The word “generally” matters because the best concentration also depends on fragment size, gel length, voltage, and running conditions; lower concentration is not the only consideration in every experiment. No agarose would provide no normal agarose-gel matrix, and a protein gel is designed for proteins rather than DNA. The principle is pore size matched to fragment size.