"My friend is carrying a pizza."
Translation:제 친구가 피자를 들고 있습니다.
있어 있어요 있습니다 doesn't change the meaning of the sentence but changes based on whom you're talking to
This question is using the present continuous tense. "Is carrying". I belive the sentence you wrote would translate to "my friend carries the pizza"
You could check out the duolingo on pc/web. There's more information along with the lessons
Again, as often is the case in the course, only one postposition is accepted, when either could be correct depending on the situation. 내가 맥주를 들거야. 내 친구는 피자를 들고 있어.
들다 has many meanings besides "carry"--hold, lift, raise. Also, a continuous thought in Korean is often expressed with simple present tense." "고 있다 is not absolutely necessary. There is no context to dictate which level of formality should be used for the verb ending or for the word "my." All we can do is hope that friend is going our way with the pizza.