Scientist set out to engineer a mutant version of LDH that would bind and reduce oxaloacetate rather than pyruvate. They made a single substitution, replacing Gln102 with an amino acid; the resulting enzyme should reduce oxaloacetate to malate. Which amino acid would be most effective at selecting the new substrate oxaloacetate

Asparagine because it is an amino acid structurally similar to the Glutamine and both have the same charge (polar neutral).

Explanation:

Glutamine (Gln or Q) is an amino acid similar to the glutamic acid, with the exception that the carboxylic acid group is replaced by an amide. In addition, glutamine, serine, threonine and asparagine are polar neutral amino acids.  

Asparagine and glutamine have a similar chemical structure due to both amino acids contain amide groups in the side chain (both are dicarboxylic amides), thereby a substitution involving these amino acids will have few deleterious effects on the resulting protein.  

A conservative substitution is a replacement where the modification is produced by substituting amino acids with similar biochemical properties (in this case, polar neutral).