To solve the given question, we need to analyze the factors affecting voltage drop in a radial distribution system. Let's go through the options and understand the concepts involved:

1. Increase in load current: The voltage drop in a radial distribution system is directly proportional to the current flowing through the system. According to Ohm's Law, the voltage drop (\(V_d\)) across a conductor can be calculated using the formula: V_d = I \times (R + jX) where: I = Load current, R = Resistance, X = Reactance, j = Imaginary unit.
   Here, an increase in load current will lead to an increase in the voltage drop across the distribution system.
2. Increase in insulation: The insulation in the wires primarily affects the safety and efficiency of the system and prevents leakage of current. It generally does not contribute directly to the voltage drop along the conductors.
3. Decrease in distance: Voltage drop is also directly proportional to the distance of the conductor. A decrease in distance would typically result in a decrease in voltage drop, contrary to what is being asked.
4. Decrease in resistance: If the resistance of the conductors decreases, it will lead to a decrease in the voltage drop. Hence, a decrease in resistance results in a reduction of the voltage drop.

Based on this analysis, the correct answer is that an increase in load current causes an increase in voltage drop.