Both statements are true, and the reason states the very condition that makes the assertion possible.

Why there are four parameter sets. A two-port has four terminal variables — V₁, I₁, V₂, I₂ — of which any two may be chosen as independent and the other two expressed in terms of them. Each choice gives a different description:

Since all four describe the same network, they must be interconvertible, and the standard conversion tables express any set in terms of any other — for example \(h_{11}=\dfrac{\Delta z}{z_{22}}\) and \(y=z^{-1}\).

Now the reason, which is the precondition for all of it. Each description takes the form of a homogeneous linear relation — a 2 × 2 matrix with no constant term. Two things would destroy that:

• an independent source inside the network would add a constant, so V₁ would not be zero when both currents were zero, and the matrix form could not hold;
• a non-zero initial condition — charge on a capacitor, current in an inductor — would add a similar constant term in the s domain.

Excluding both is exactly what allows the network to be characterised by four numbers, and hence what allows the four sets to be related to one another. So (R) explains (A) and the code is 1.

What is not excluded : dependent (controlled) sources are perfectly permissible, because their value is proportional to a terminal variable and so contributes to the matrix rather than to a constant term. Their presence simply makes the network non-reciprocal, so \(z_{12}\ne z_{21}\) and the matrix loses its symmetry. That is precisely how a transistor is modelled.

A caution on existence : the four sets are interconvertible but not all of them always exist. A network whose z matrix is singular has no y parameters, and an ideal transformer has neither z nor y but perfectly good ABCD parameters — which is a further reason for keeping all four in the toolkit.

Hence, both (A) and (R) are true and (R) is the correct explanation of (A).