Which observation would provide the strongest evidence that most noncoding DNA is slightly deleterious?

The correct option is species with smaller population sizes have more non-coding DNA.

Explanation:

Non-coding DNA can be described as the DNA which does not code for anything and hence, is not involved in the formation of proteins.

In smaller populations, we can see that most of the genome of species is non-functional. Studies in these populations show that even if some part of the genome is deleted, no changes occur in the organism as these regions do not code for any specific proteins.