The problem . . . . . . . . not to one of internal. A. are B. am C. is D. were

The correct answer is: C. is

The sentence is in the present tense, so the verb “to be” must be conjugated in the present tense as well. The subject of the sentence is “problem,” which is singular, so the correct verb form is “is.”

The other options are incorrect because they are not conjugated in the present tense or because they do not agree with the subject of the sentence.

• Option A: “are” is the plural form of the verb “to be.” It is used with plural subjects.
• Option B: “am” is the first person singular form of the verb “to be.” It is used with the pronoun “I.”
• Option D: “were” is the past tense form of the verb “to be.” It is used with past tense subjects.

I hope this helps! Let me know if you have any other questions.