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 A - 'Have' is a plural verb, which is used with plural subjects. Here, 'Smita' is a singular subject, hence we can't say that 'Smita have been teaching English for seven years now.' Thus option A is incorrect.
Option B - The word 'now' in the sentence explains to us that the sentence is in present tense. 'Had been teaching' is past perfect continuous tense. Hence, we can't use 'had' here. Thus option B is incorrect.
Option C - 'Taught' is the past form of the verb 'teach'. As the sentence is in present tense, we can't use past tense. Thus, option C is incorrect.
Option D - 'Smita' is a singular subject and hence we need a verb in its singular form. 'Has been teaching' can be used here because the action started in the past and continues in the present. Hence, we can conclude 'Smita has been teaching English for seven years now' is correct as the subject and the verb agree in the sentence. Thus option D is the correct answer.