Reciprocal of admittance is impedance.

Admittance:
Admittance is the measure of how easily alternating current (AC) flows through a circuit. It is the reciprocal of impedance. Admittance is represented by the symbol Y and is measured in siemens (S).

Impedance:
Impedance is the total opposition that a circuit presents to the flow of AC. It is a combination of resistance, inductive reactance, and capacitive reactance. Impedance is represented by the symbol Z and is measured in ohms (Ω).

Reciprocal:
Reciprocal is the mathematical operation that gives the multiplicative inverse of a number. In the context of admittance and impedance, taking the reciprocal of admittance gives the impedance.

Explanation:
When an AC signal passes through a circuit, it encounters different types of opposition. Resistance is the opposition to the flow of current in a circuit, and it is represented by the symbol R. Inductive reactance (XL) and capacitive reactance (XC) are the opposition caused by the inductors and capacitors in the circuit, respectively.

The total opposition to the flow of AC in a circuit is given by impedance (Z), which is a combination of resistance, inductive reactance, and capacitive reactance. Impedance can be calculated using the following formula:

Z = R + j(XL - XC)

Where j is the imaginary unit (√(-1)).

Admittance (Y) is the reciprocal of impedance (Z). Mathematically, it can be expressed as:

Y = 1/Z

Taking the reciprocal of impedance gives the admittance of the circuit. Admittance is useful in analyzing AC circuits as it provides a measure of how easily current can flow through the circuit.

Therefore, the correct answer is option 'D' - Impedance.