Which of the following compounds is isomeric with 2, 2, 4, 4- tetramethylhexane? (a) 3-ethyl -2, 2- dimethylpentane (b) 4-isopropylheptane (c) 4-ethyl-3-methyl-4-n propyloctane (d) 4, 4-diethyl-3-methylheptane