A decrease in the current of a series circuit can be caused by:

In a series circuit, the current is determined by the total voltage applied to the circuit and the total resistance in the circuit, according to Ohm's Law, which states that current (I) equals voltage (V) divided by resistance (R) (I = V/R).

When resistance increases, the overall resistance of the circuit becomes greater. Since the voltage remains constant in this scenario, a higher resistance will result in decreased current through the circuit. This relationship is fundamental in understanding how electrical circuits function.

Increasing the load, for instance, typically involves adding more resistance to the circuit, which would lower the current as well. However, since the question specifically focuses on the direct impact of resistance, the correct focus is on resistance increase as the definitive cause of current decrease in a series configuration.

Other choices, like increasing the voltage, would lead to increased current, while changing the frequency and decreasing the load do not specifically lead to a decrease in current; rather, they can potentially lead to increased current depending on the context and circuit configuration. Thus, resistance increase is the primary factor that directly results in decreased current in a series circuit.