Final answer:
The correct sentence is 'Be a bit more careful.' The verb 'Be' is followed by the adverbial 'a bit more careful' to form a complete, imperative sentence.
Step-by-step explanation:
The correct grammatically sentence from the options provided is: 'Be a bit more careful.' When arranging the words to form a grammatically correct sentence, you must follow the standard English sentence structure which is: subject + verb + object. In this case, 'Be' is the imperative verb making the subject an implied 'you', and 'a bit more careful' is the phrase that serves as the adverb modifying the verb to provide intensity. The rest of the words ('a', 'bit', 'more', 'careful') should be arranged around this structure to convey the complete thought effectively.
To Be
In the given sentence, 'be' serves as the main verb. As explained in the lesson on forms of 'to be', this verb is often followed by adverbials, adjectives, or noun phrases. 'A bit more careful' acts as the adverbial modifying the verb 'be' to further describe the manner in which something should be done, in this case, being careful.