Final answer:
In a series circuit, all the resistors are connected end-to-end, providing only one path for current, which flows through each resistor sequentially. This is the configuration where resistors are connected in series.
Step-by-step explanation:
In a series circuit, all the resistors connected in such a way that there is only one path for the charge to flow, meaning the same current flows through each resistor sequentially.
Therefore, the correct answer to the question 'In which circuit are all the resistors connected in series?' is option (a) Series circuit. In contrast, a parallel circuit has its resistors connected in a manner where the voltage across each resistor is the same, but different currents can flow through each resistor.
A combination circuit has elements of both series and parallel configurations, and an open circuit represents a disconnection in the circuit, preventing any current flow.