Explanation: Electromagnetic field strength (measured in microvolts per meter) decreases as the distance from the source increases. For an ideal propagation path, field strength is inversely proportional to the distance ($E \propto 1/D$). This means if the distance doubles, the field strength halves. In this problem, the distance doubles from 100 miles to 200 miles. Consequently, the field strength will be halved. Initial field strength = 100 microvolts per meter New distance / Old distance = 200 miles / 100 miles = 2 New field strength = Initial field strength / 2 = 100 microvolts / 2 = 50 microvolts. Therefore, the field strength at 200 miles is 50 microvolts per meter. Options B, C, and D are incorrect as they do not reflect this inverse linear relationship.