Increasing ratios of oxygen-18 to oxygen-16 in glacial ice indicate a. advancing ice sheets b. warming temperatures c. increased volcanic emissions d. uplift of land surfaces

Increasing ratios of oxygen-18 to oxygen-16 in glacial ice indicate b. warming temperatures.
In glacial ice there are cyclical variations in the ratio of the abundance of oxygen-18 to the abundance of oxygen-16.
The ¹⁸O/¹⁶O ratio can show water temperature, because more energy is required to vaporize water with oxygen-18 than water with oxygen-16, so because of increase of temperature water with oxygen-16 diffuse and vaporize faster and water with more oxygen-18 stays in ice.