Which reagent is preferred for converting 2-methyl-1-propanol to 2-methylpropanal?

Well you are going from a primary alcohol to an aldehyde so you are doing an oxidation. That eliminates b, c, and d. That leaves a or e. CrO3/H+, K2CrO7/H+ and KMnO4 are all strong oxidizers so they will oxidize the primary alcohol to a carboxylic acid which of course is not the product. PCC however, is a more mild oxidizer that will stop at the aldehyde level of oxidation giving you 2-methylpropanal. So, long story short....the answer is A.