Which result would indicate a positive Indole test?

The positive Indole test is indicated by red ring formation at the top of the broth culture after the addition of Kovac's reagent. This reaction occurs when the amino acid tryptophan is metabolized by certain bacteria into indole, which then reacts with Kovac's reagent to produce a red color. This is a key biochemical test used to differentiate between various Gram-negative bacteria, especially within the Enterobacteriaceae family.

The other options relate to different types of biochemical tests or reactions. Blue coloration typically indicates a positive result in tests that assess the presence of certain amino acids or pH changes, while yellow broth may indicate a fermentation test that shows acid production. Gas bubble formation is often associated with fermentation tests but does not indicate indole production specifically. Thus, the formation of a red ring is the hallmark of a positive Indole test, clearly distinguishing it from other biochemical reactions.