The correct option is B The Met. office says that it will rain heavily tomorrow.
The given sentence is in direct speech. When the reporting verb 'says' is in present tense, the tense of the direct speech will not change.
Hence, the reported speech is: The Met. office says that it will rain heavily tomorrow.
Hence option A is correct.
Option B: The tense of the speech must not change, according to the rule mentioned above. In this option, 'will' has changed to its past form 'would'. Hence B is incorrect.
Option C: Here, the verb form has been changed 'would be raining', even though it's supposed to remain the same. Hence the option is incorrect.
Option D is incorrect because the answer lies in A.