The context provides the answer directly in section 6.4. For neopentane (H₃C-C-CH₃), all hydrogen atoms are equivalent, so replacement of any hydrogen gives the same product - a single monochloride. For n-pentane, the equivalent hydrogens are grouped as a, b and c (CᵃH₃CᵇH₂CᶜH₂CᵇH₂CᵃH₃), giving three isomeric monochlorides. For isopentane, the equivalent hydrogens are grouped as a, b, c and d (CᵃH₃CᵇHCᶜH₂CᵈH₃), yielding four isomeric products.