Explanation:

Pre-monsoon showers, locally known as 'Mango Showers' (Amra Varsha), are common in the southern states of Kerala and Karnataka. These rains are crucial for the early ripening of mangoes and are also beneficial for coffee plantations. Therefore, option B is correct. Option A, Rajasthan and Gujarat, are arid/semi-arid regions where such specific pre-monsoon phenomena for mangoes are not prominent. Option C, West Bengal and Assam, experience pre-monsoon thunderstorms known as 'Nor'westers' or 'Kal Baisakhi', which are different from 'Mango Showers'. Option D, Uttar Pradesh and Bihar, also experience pre-monsoon activities but not the specific 'Mango Showers' phenomenon associated with mango ripening in the south. Thus, A, C, and D are incorrect.