You have two purified samples of protein Y: the wild-type (nonmutated) protein and a mutant version ...

You have two purified samples of protein Y: the wild-type (nonmutated) protein and a mutant version with a single amino acid substitution.
 
  When washed through the same gel-filtration column, mutant protein Y runs through the column more slowly than the normal protein. Which of the following changes in the mutant protein is most likely to explain this result?
  (a) the loss of a binding site on the mutant-protein surface through which protein Y normally forms dimers
  (b) a change that results in the mutant protein acquiring an overall positive instead of a negative charge
  (c) a change that results in the mutant protein being larger than the wild-type protein
  (d) a change that results in the mutant protein having a slightly different shape from the wild-type protein