The correct option is C Either Ridhima or Pooja has won.
In order to choose the sentence with the correct subject and verb agreement, let's understand what subject-verb agreement is. If the subject of the sentence is singular, the verb must be in its singular form; and if the subject of the sentence is plural, the verb must be in its plural form.
Option A - 'Either Ridhima or Pooja have won' means one has won. The conjunction 'either-or' means any one of the two nouns or subject. Hence, there is just one main subject in the sentence, and the verb should be in its singular form too. 'Have' is a plural verb form and can't be used with a singular subject. Thus, option A is incorrect.
Option C - 'Ridhima and Pooja is winner' means both of them are winners. The conjunction 'and' is used in case of more than one subject or noun. As the subject in the given sentence is plural, the verb should be in its plural form as well. However, 'is' is a singular auxiliary verb and can't be used with plural subject. Hence, option C is incorrect.
Option D - 'Ridhima or Pooja are winner' is incorrect. The conjunction 'or' is used to indicate a singular subject/ choice. 'Are' is a plural verb form and can't be used with a singular subject. Hence, option D is incorrect.
Option B - 'Either Ridhima or Pooja has won' is grammatically correct as both the subject and the verb agree with one another in the sentence. 'Either-or' is used to indicate a choice/ singular subject in a sentence. 'Has' is a singular verb form and hence, it can be used with a singular subject. Thus option B is the correct answer.