tion:=>In order to select the correct FORM of the verb to complete the sentence, let's understand the subject-verb AGREEMENT. Subject and verb should agree with each other, i.e., if the subject is singular, the verb should also be in its singular form; and if the subject is plural, the verb should also be in its plural form.Option B - Though 'mathematics' has an 's' in the END, it's not a plural word. It is the name of a subject, and hence it is a singular subject. We MUST fill in the blank with a singular verb. As 'are' is a plural verb form, we can't use it here. Hence, option B is incorrect.Option C - 'Were' is a plural verb form. As explained above, 'mathematics' is a singular subject and so, we NEED a singular verb form. Hence, option C is incorrect.Option D - 'Been' is a singular as well as a plural auxiliary verb which is used with both singular as well as plural subject. 'Been' is always accompanied by a helping verb like - has, have or had. It is often used in perfect o perfect continuous tenses. Hence, option D is incorrect.Option A - Since 'mathematics' is a singular subject, 'is' is a singular verb form. As both the subject and the verb agree in the sentence, we can say - 'Mathematics is my favourite subject.' This is your answer...