The correct option is
C I am writing
In the given sentence, 'is' indicates that the tense of the sentence is present tense. Thus, the answer must be in present tense. Further, the sentence does not have a noun or pronoun that introduces the subject. Thus, the answer must have a noun/pronoun. Also, the sentence is missing a main verb and thus, the answer must have a complete verb.
Option A is incorrect. 'of writing' does not have a pronoun/noun that introduces the subject. Also, it does not have a complete verb. 'writing' is a gerund and not a complete verb.
Option B is incorrect. 'for me write' is grammatically incorrect. Here, the subject and the verb do not agree with each other and the phrase itself does not make any sense. Thus, it is incorrect.
Option C is incorrect. 'that I had written' is in the past perfect tense and not the present tense. Thus, it does not have the required tense.
Option D is correct. 'I am writing' is in the present continuous tense. 'I' is a pronoun that introduces the subject and 'am writing' is a complete verb. Thus, it meets all the required conditions and is the correct answer.