If bubbles are observed during the mixing of two liquids and the mixture has not reached its boiling point, the most likely explanation for what is inside the bubbles is one of the liquids that changed to a gas.
This can happen due to several reasons, such as a change in pressure, temperature, or due to a chemical reaction that produces gas. However, since the question specifies that the mixture did not reach its boiling point, the formation of bubbles is typically indicative of a volatile component that vaporizes or gas that evolves from a reaction. The other options (a mixture of the liquids that reacted, air released from the liquids, or a new substance that formed) are less likely in this context.