The correct option is
D am talking
The sentence talks about an event that is happening in the present, as can be inferred from the word 'now'.
Present continuous tense describes an ongoing action that is happening at the same time the statement is written. This tense is formed by using “am/is/are” with the verb form ending in “-ing”. For example - I am reading this book at the moment.
'I is' is incorrect and 'I am' is correct axiomatically, that is its a rule and not a consequence. Therefore option C is incorrect.
'Are' is not used with first person singular forms. Therefore option B is incorrect.
'Talk' as a verb can only be used in sentences that are generalized or habitual. For Eg. I talk to my teacher everyday. They talk a lot. However 'talk' cannot be used in present continuous form, therefore option A is incorrect.
Option B is in the correct form of present continuous therefore is correct.