Out of the four options, option D is correct. When the subject is separated from the verb by the word 'as well as', singular verb is used if the subject is singular. Here, 'rice' and 'wheat' are uncountable nouns. Uncountable nouns are in singular form and always take a singular verb. 'Were' and 'are' are plural verbs hence, options B and C are incorrect according to the rule. 'Has been' is a singular verb thus, option D is correct.