Identify a mobile source of food.

The correct option is Deer.

Explanation:
In an ecosystem, food sources can be categorized as either mobile or immobile depending on whether they have the ability to move independently.

1. Mobile Source of Food: This refers to living animals that can move around freely in their environment. A Deer is a living herbivore that moves throughout its habitat, making it a mobile source of food (prey) for carnivores and omnivores.
2. Immobile Sources of Food: Plants such as Grass and Shrubs are anchored into the soil by their roots and cannot relocate. Similarly, a Dead goat is non-living matter and lacks the ability to move.

Therefore, the Deer is the only mobile source of food among the choices provided.