The correct answer is “May I use your cell phone as I have forgotten mine?”.
The word “may” is used to ask for permission. In this case, the speaker is asking the listener for permission to use their cell phone.
The word “should” is used to give advice or make a suggestion. In this case, the speaker is not giving advice or making a suggestion, they are asking for permission.
The word “would” is used to express a wish or desire. In this case, the speaker is not expressing a wish or desire, they are asking for permission.
The word “do” is used to make a request or demand. In this case, the speaker is not making a request or demand, they are asking for permission.
Therefore, the correct answer is “May I use your cell phone as I have forgotten mine?”.