To analyze a closed-loop circuit containing an ideal operational amplifier, understanding the basic assumptions regarding the behavior of an ideal operational amplifier is crucial. One key assumption involves the currents at the input terminals.

1. Operational Amplifier Characteristics:

An ideal operational amplifier is characterized by infinite input impedance. This means that it prevents any current from flowing into the input terminals, both inverting (-) and non-inverting (+).

1. Implications of Infinite Input Impedance:

Due to infinite input impedance, the current into the inverting and non-inverting input pins is effectively zero in an ideal operational amplifier.

1. Assumptions for Ideal Operation:

One of the basic assumptions when analyzing circuits with an ideal operational amplifier is that no current flows into either input terminal.

1. Comparison with Given Options:
   • The correct option — "No current flows into either input terminal" — aligns with the fundamental characteristic of an ideal op-amp.
   • Other options contradict the principle of infinite input impedance:
     • "A small bias current flows into each terminal" implies non-zero input current, which is incorrect for an ideal op-amp.
     • "Current flows only into the non-inverting terminal" indicates asymmetry in input behavior, which is false for an ideal op-amp.
     • "Equal and opposite currents flow into both terminals" suggests balanced but non-zero input currents, also incorrect for an ideal op-amp.

Thus, considering the aforementioned points, we conclude that the suitable assumption for a closed-loop circuit with an ideal operational amplifier is that no current flows into either input terminal.