Answer / Explanation:
Coordination numbers, grain growth in ice, and till deformation.
The Web of Science lists 5 papers that Dr. Alley helped write and that have 1986 publication dates. Dr. Alley was studying why some parts of the Antarctic ice sheet move rapidly, and helped learn that deformation of a special type of mud beneath, called till, was involved. He also was studying the physical properties of ice cores, including why and how some crystals or grains in the ice get bigger over time. Some of the physical properties of the ice cores are related to how many different grains are touching each other, which is the coordination number.
Later, Dr. Alley used his knowledge of physical properties in ice to learn how old ice cores are, and to help learn about climate history from them. The other possibilities listed are all things that he studied, some rather closely related, but that were not published in 1986.
