A: Chlorine actually gives a higher bond enthalpy than iodine, because a chlorine is more electronegative = stronger attraction to electrons in the bond.
B: The statement is correct, but that fact would predict that the more polar C-Cl bond would be better at attracting the nucleophilic OH- ions, leading to higher rate of reaction with chlorine.
C:Plain wrong. C-Cl is more polar due to chlorine's electronegativity, so it has larger + and - charges.
D:The key is that because iodine creates less polarity, it forms a weaker bond which can be broken off with less energy. This outweighs the effect of decreased charge on the adjacent carbon. The thing is, there's no way we students could actually work out which is a larger factor, so we just have to memorise that it is the case.
C and D are the only answers that could be logically applied as an explanation for the increased rate if we assume the statements are true. C is an incorrect fact though, so the answer has to be D.
Essentially, its a nasty question conceptually as its entire premise is you having memorised that for these reactions, bond enthalpy is more important than bond polarity. However by process of elimination of wrong or unapplicable statements you can arrive at D anyway.