The correct answer is: E. No error
The sentence is grammatically correct. The word “must” is used to express obligation or necessity. In this sentence, it is used to express the idea that motorists are required to be very cautious while driving along the highways in order to save petrol. The word “cautious” means careful and watchful, and it is appropriate to use this word in this context because motorists need to be careful and watchful when driving in order to avoid accidents. The word “while” is used to introduce a clause that describes the time at which something happens. In this sentence, it is used to introduce a clause that describes the time at which motorists need to be cautious, which is while they are driving along the highways. The word “highways” is a plural noun that refers to roads that are designed for high-speed travel. In this sentence, it is used to refer to the roads that motorists are driving along.
Here are some additional details about each option:
- Option A: “In order to save petrol” is a prepositional phrase that introduces the main clause of the sentence. It is grammatically correct.
- Option B: “motorists must have to” is a verb phrase that expresses the idea that motorists are required to do something. It is grammatically correct.
- Option C: “be very cautious” is an infinitive phrase that describes the way in which motorists need to behave. It is grammatically correct.
- Option D: “while driving along the highways” is a prepositional phrase that introduces a clause that describes the time at which something happens. It is grammatically correct.
Therefore, the sentence is grammatically correct and there is no error.