Answer:
b. I am typing a letter, and she is talking on the phone.
Step-by-step explanation:
The statement I am typing a letter, and she is talking on the phone is not properly punctuated and reason being that the comma(,) before the 'and' word should not be there since 'and' already serve as a conjunction and what conjunction does is to combine two statements together in a sentence. The comma that is present there is creating a pause while reading the sentence which is not suppose to be so because the presence of 'and' word there will cause the sentence to be pronounced at once without any pause.
The correct punctuation is - I am typing a letter and she is talking on the phone.