Final answer:
The number of electric-field lines is proportional to the charge's magnitude. For a charge four times greater than a +5 nC charge, which has five lines, there would be 20 field lines.
Step-by-step explanation:
The number of electric field lines emanating from a charged object is directly proportional to the magnitude of the charge. In this case, if a +5 nC charge has five electric field lines, then a +20 nC charge would have four times the magnitude of charge and thus four times the number of electric field lines. The number of electric-field lines related to a charge is proportional to the magnitude of that charge. Since the question states that five electric-field lines come out of a +5 nC charge, we can deduce that the number of field lines is directly proportional to the charge. For a +20 nC charge, which is four times greater than the +5 nC charge, we would expect four times the number of field lines. Hence, the correct answer is d. 20 field lines.