The correct answer is: A. had better
The sentence “We had better start now to work for our examination” is an example of a modal verb construction. Modal verbs are used to express necessity, possibility, or permission. In this case, the modal verb “had better” is used to express a strong recommendation or advice. The sentence means that it is very important for us to start working on our examination now.
The other options are incorrect because they do not express the same level of urgency or recommendation. The verb “better” is used to express a suggestion or recommendation, but it is not as strong as “had better.” The verb “has better” is not a correct English construction. The verb “are better” is used to compare two or more things, but it does not make sense in this sentence.
In conclusion, the correct answer to the question “We …….. start now to work for our examination” is: A. had better.