In an ecosystem, organisms are classified into different trophic levels based on their feeding habits and position in the food chain. The primary consumer is the first level of consumers in the food chain, which feeds directly on producers (plants).

Option 1: Caterpillar: Caterpillars are herbivores that primarily feed on plants. They consume leaves, stems, and other plant parts, making them an example of a primary consumer. This is the correct answer.

Option 2: Crabapple tree: Crabapple tree is a plant, not a consumer. It is a producer and would be categorized under the autotrophs.

Option 3: Frog: Frogs are usually considered secondary consumers as they feed on insects and other small animals. They do not directly consume producers, so they are not primary consumers.

Option 4: Sparrowhawk: Sparrowhawks are birds of prey and are higher up in the food chain. They typically feed on small birds and other animals, making them tertiary or quaternary consumers.

Based on these explanations, option 1 (Caterpillar) is the correct answer as it represents the primary consumer category by directly feeding on plants.