Which of the following sentences is written correctly?

A When properly designed, an automobile can achieve great fuel efficiency.
B. When properly designed an automobile can achieve great fuel efficiency.
C. When properly designed, great fuel efficiency can be achieved by an automobile.
D. When properly designed great fuel efficiency can be achieved by an automobile

To determine which sentence is written correctly, we should consider proper grammar and punctuation.

Sentence A: "When properly designed, an automobile can achieve great fuel efficiency." This sentence is written correctly. It includes a comma after the introductory phrase "When properly designed."

Sentence B: "When properly designed an automobile can achieve great fuel efficiency." This sentence is missing a comma after the introductory phrase. It should be "When properly designed, an automobile can achieve great fuel efficiency." Therefore, it is not written correctly.

Sentence C: "When properly designed, great fuel efficiency can be achieved by an automobile." This sentence is grammatically correct, but it has a different structure compared to the other options. It rearranges the sentence to focus on great fuel efficiency rather than the automobile achieving it. While it is grammatically correct, it may not fit the context or intention of the original sentence.

Sentence D: "When properly designed great fuel efficiency can be achieved by an automobile." This sentence is missing a comma after the introductory phrase "When properly designed." It should be "When properly designed, great fuel efficiency can be achieved by an automobile." Therefore, it is not written correctly.

So, the correct sentence is A. "When properly designed, an automobile can achieve great fuel efficiency."