Parts of the following sentence have been given as options. Select the option that contains an error.
My neighbours were serious injured in the car accident.

The correct option that contains a grammatical error is "serious injured".

Step-by-step Explanation:
1. Let's analyze the grammar of the sentence: "My neighbours were serious injured in the car accident."
2. In this sentence, "injured" is a past participle acting as an adjective (or part of the passive verb phrase "were injured") to describe the state of the neighbours.
3. To modify an adjective or a verb (like "injured"), we must use an adverb rather than an adjective.
4. "Serious" is an adjective, whereas "seriously" is the corresponding adverb.
5. Therefore, "serious" must be changed to the adverb "seriously" to correctly modify "injured". The grammatically correct sentence should be: "My neighbours were seriously injured in the car accident."