Introduction:
A balanced 3-phase system refers to a system where the three phases (A, B, and C) have equal magnitudes and are spaced 120 degrees apart. In such a system, the currents flowing in the three phases can be classified into different sequences: positive sequence, negative sequence, and zero sequence.

Explanation:
The correct answer to the given question is option 'B', which states that a balanced 3-phase system consists of positive-sequence currents only. Let's understand why this is the correct answer.

Positive sequence:
The positive-sequence currents refer to the currents that have the same magnitude and phase sequence as the voltage source. In a balanced 3-phase system, the positive-sequence currents have the following characteristics:

1. Magnitude: The magnitude of the positive-sequence currents is the same in all three phases. This is because the system is balanced, and the loads connected to each phase are equal.

2. Phase sequence: The positive-sequence currents have the same phase sequence as the voltage source. This means that the phase A current leads the phase B current by 120 degrees, and the phase B current leads the phase C current by 120 degrees.

3. Symmetry: The positive-sequence currents are symmetrical with respect to the zero line. This means that the sum of the positive-sequence currents in the three phases is zero.

Other sequences:
While a balanced 3-phase system consists of positive-sequence currents, it may also have other types of currents:

1. Negative sequence: The negative-sequence currents have the same magnitude as the positive-sequence currents but a different phase sequence. In a balanced system, the negative-sequence currents are typically caused by unbalanced loads or faults. However, in a perfectly balanced system, the negative-sequence currents are absent.

2. Zero sequence: The zero-sequence currents refer to the currents that flow in the neutral conductor of a 3-phase system. In a balanced system, the zero-sequence currents are typically caused by unbalanced loads or faults. However, in a perfectly balanced system, the zero-sequence currents are absent.

Conclusion:
In conclusion, a balanced 3-phase system consists of positive-sequence currents only. The positive-sequence currents have the same magnitude and phase sequence as the voltage source, making them symmetrical and balanced. The presence of negative-sequence currents or zero-sequence currents indicates an unbalanced condition or a fault in the system.