The most likely explanation for a child exhibiting a heritable trait that neither parent exhibits is:
The trait is recessive, and the child inherited a recessive allele from each parent.
This explanation is consistent with the concept of recessive traits in genetics, where two parents can carry a recessive allele (which they do not express phenotypically) and pass it on to their child, resulting in the child expressing the trait. The other explanations, while possible, are less likely in this scenario. For example, a mutation would typically suggest a new or novel trait rather than a heritable trait, and inheritance from a grandparent would not apply if neither parent exhibits the trait.