Final answer:
Blisters on a flat roof can be caused by improperly overlapping seams, poor ventilation, and insufficient insulation, which allow air and moisture to become trapped, leading to the formation of blisters.
Step-by-step explanation:
A blister on a flat roof can be caused by a variety of factors. When a blister occurs, it generally means that there are pockets of air or moisture trapped between the layers of the roofing material or between the roofing material and the substrate. While overlapping seams improperly may allow water and air to penetrate the roofing layers, other potential causes like poor ventilation can prevent moisture underneath the roof from escaping, which can also result in blisters forming. Insufficient insulation might lead to temperature differences that cause condensation, contributing to blister formation. Therefore, the correct answer would be d. All of the above.