Answer:

To understand why the constituent 'R alone' cannot be a part of the circuit, let's first understand the concept of phase difference between alternating current (AC) and electromotive force (EMF).

Phase Difference:
The phase difference between the AC and EMF in an AC circuit is the difference in the timing of their peaks or zero-crossings. It is denoted by the Greek letter phi (φ) and is measured in radians.

Phase Difference of π/2 (or 90 degrees):
A phase difference of π/2 means that the AC current and EMF are out of phase by 90 degrees. This means that when the EMF reaches its maximum (or minimum) value, the AC current will be zero, and vice versa.

Analysis of Options:
Let's analyze each option to determine whether it can be a part of the circuit with a phase difference of π/2:

a) L, C: In an AC circuit, a combination of inductance (L) and capacitance (C) can produce a phase difference of π/2. This is known as a resonant circuit. Therefore, option 'a' can be a constituent of the circuit.

b) L alone: Inductor alone cannot create a phase difference of π/2. It can only introduce a phase shift of 90 degrees if it is combined with a capacitor. Therefore, option 'b' cannot be a constituent of the circuit with a phase difference of π/2.

c) C alone: Similarly, a capacitor alone cannot create a phase difference of π/2. It can only create a phase shift when combined with an inductor. Therefore, option 'c' cannot be a constituent of the circuit with a phase difference of π/2.

d) R alone: A resistor alone does not introduce any phase shift in the circuit. It only dissipates energy in the form of heat. Therefore, option 'd' cannot be a constituent of the circuit with a phase difference of π/2.

Conclusion:
Based on the analysis, the constituent 'R alone' cannot be a part of the circuit with a phase difference of π/2.