The following sentence has been split into four segments. Identify the segment that contains a grammatical error.
It is a cheap reasonably / restaurant in this part of the town / where you can eat / delicious food.

The correct option containing the grammatical error is It is a cheap reasonably.

Step-by-Step Explanation:

1. Let us analyze the structure of the given sentence segments:
- "It is a cheap reasonably / restaurant in this part of the town / where you can eat / delicious food."

2. Look closely at the noun phrase "a cheap reasonably restaurant":
- Here, "restaurant" is a noun.
- The words modifying "restaurant" are adjectives.
- "cheap" is an adjective modifying "restaurant".
- "reasonably" is an adverb. An adverb is used to modify verbs, adjectives, or other adverbs, but not directly modify a noun in this context.

3. To modify the adjective "cheap", the adverb "reasonably" should come before it (e.g., "a reasonably cheap restaurant"), meaning a restaurant that is reasonably inexpensive.
Alternatively, if two adjectives are used to describe the noun, both must be in adjective form (e.g., "cheap and reasonable").

4. Therefore, the phrase "cheap reasonably" is grammatically incorrect. The correct usage should be "a reasonably cheap restaurant".

Hence, the error lies in the segment "It is a cheap reasonably".