We ........ start now to work for our examination.

The options given are different forms of the verb "better".

Option A: "had better" is a modal verb phrase that means "should" or "ought to". It expresses a strong recommendation or advice.

Option B: "better" is an adverb and cannot be used with the verb "start" in this context. It needs an auxiliary verb like "had" to form a complete verb phrase.

Option C: "has better" is grammatically incorrect because "has" is a singular verb that doesn't agree with the plural implied subject "we".

Option D: "are better" is also incorrect because "are better" doesn't fit the context of starting work. It suggests a comparison of being better than something else.

Therefore, the correct option is A ("had better") because it accurately conveys the meaning of a strong suggestion to begin working for the examination.