After mixing a heat-killed, phosphorescent (light-emitting) strain of bacteria with a living, nonphosphorescent strain, you discover that some of the living cells are now phosphorescent. Which observation (s) would provide the best evidence that the ability to phosphoresce is a heritable trait?

Phosphorescence in descendants of the living cells

Explanation:

Bacteria have the ability to take up the gene from other bacteria or surrounding. So when heat-killed phosphorescent (light-emitting) strain of bacteria is mixed with a non-phosphorescent strain than there is a chance that the gene that encodes for phosphorescence is transferred into some non-phosphorescent strain.

So those bacteria which get the gene will have ability to phosphorescent and can pass its gene to its descendant. So if the phosphorescence ability passes to offspring then it would provide evidence that the ability to phosphoresce is a heritable trait.