ST 1 and ST 2 are correct — option (D), as recorded in the supplied key.

Statement 1 is certainly true. The admittance and impedance matrices of a two-port are inverses of one another :

\([Y]=[Z]^{-1}\)

and the conversion is explicit. Writing \(\Delta Z=Z_{11}Z_{22}-Z_{12}Z_{21}\),

\(Y_{11}=\dfrac{Z_{22}}{\Delta Z},\quad Y_{12}=\dfrac{-Z_{12}}{\Delta Z},\quad Y_{21}=\dfrac{-Z_{21}}{\Delta Z},\quad Y_{22}=\dfrac{Z_{11}}{\Delta Z}\)

So whenever the Z-parameters of a network are known and the matrix is non-singular, the Y-parameters follow by arithmetic alone with no further measurement.

Statement 2 follows from statement 1 on the key’s reading. If every Y-parameter can be computed from the Z-parameters, then a separate defining experiment for the Y set is not necessary — the same information is already present in the Z description, merely expressed in another basis. The two parameter sets describe one network, not two.

The qualification worth carrying, because it is the reason a careful candidate hesitates over ST 2. The conversion requires \(\Delta Z\neq 0\), and for some networks it is exactly zero — a series impedance in the through path has no finite Y description, and a shunt element has no finite Z description. Those cases are why all six parameter sets exist :

The practical reason for defining Y separately is convenience rather than necessity: measuring Y requires short-circuit terminations while Z requires open-circuit ones, and at high frequency a good short is far easier to realise than a good open, which is why Y and S parameters dominate RF work.

Note. Read strictly — as a claim that the Y set never needs independent definition — ST 2 fails for the singular cases above, which would give option (A). The answer stored here follows the supplied key.