Answer: C. Unfavorable geometry
Step-by-step explanation: When collision occurs between two reactants in order to make a reaction possible there are 3 factors which are responsible.
a) Orientation factor
b) Energy factor
c) rate of collision
Thus out of the given options, unfavorable geometry is the correct one as temperature and concentration as well as surface area will have very little effect on the reaction.
If the geometry of the reactant is not complementary then the reaction would not lead successfully.