A large amount of dust and ash from a recent volcanic eruption settles in the region, which is most likely to survive, deer, fish in a pond, grass or trees

Volcanic Ash and dust would cover the surface of the pond. This would prevent the growth of planktons which serves as food for them. Sunlight would not be able to penetrate the pond to furnish it with a very important ingredient for photosynthesis. As we know that most fishes are heterotrophs.

For grasses and trees, the dust and Ash particles would cover the leaves which is the site of photosynthesis. Plants are autotrophs that manufactures their own food through sunlight and carbondioxie. Most of the plants would die off since they can't remove the Ash and dust covering on them.

A deer is an heterotroph and a motile animal. It can easily leave that region and source for food elsewhere in the ecosystem.