The answer is d, but I'm unsure as how to get it.

5. In the case of anionic polymerization using organometallic initiator, Abby was able to get polystyrene with a number average molecular weight of 31,200g/ mole. Please estimate the outcome of the new polymer if she double the initial monomer concentration and quadruple the initiator concentration from the original settings with all the other conditions/parameters and conversion being the same? (a) Mn=31,200moleg and polydispersity =1.007 (b) Mn=31,200moleg and polydispersity =1.50 (c) Mn=15,600moleg and polydispersity =1.50 (d) Mn=15,600moleg and polydispersity =1.007 (e) Mn=7,800moleg and polydispersity =1.007 (f) Mn or polydispersity do not depend on monomer or initiator concentration