In the field of electronics and communication engineering, short circuit forward transfer admittance is an important concept. It refers to the admittance of a two-port network when the output port is short-circuited and the input port is open-circuited. Among the given options (a), (b), (c), and (d), option (C) is regarded as the short circuit forward transfer admittance.

To understand why option (C) is the correct answer, let's first understand the meaning of the terms used in the options. The terms y11, y12, y21, and y22 correspond to the parameters of a two-port network called the y-parameter or admittance parameters. These parameters relate the current and voltage at the input and output ports of the network.

Now, let's break down each option and explain their meanings:

a) y11: This parameter represents the input admittance when the output port is short-circuited. It is not the short circuit forward transfer admittance.

b) y12: This parameter represents the admittance between the input and output ports when the output port is short-circuited. It is not the short circuit forward transfer admittance.

c) y21: This parameter represents the output admittance when the output port is short-circuited. This is the short circuit forward transfer admittance and is the correct answer.

d) y22: This parameter represents the admittance between the output and input ports when the output port is short-circuited. It is not the short circuit forward transfer admittance.

Therefore, option (C) - y21 is regarded as the short circuit forward transfer admittance. This parameter helps in understanding the behavior and characteristics of a two-port network when the output port is short-circuited.

Understanding and analyzing the short circuit forward transfer admittance is crucial in designing and analyzing electronic circuits and systems. It provides valuable information about the network's input and output characteristics, which aids in circuit analysis and design considerations.