Introduction:
An ideal operational amplifier (op-amp) is a theoretical model that simplifies the analysis of op-amp circuits. It assumes certain properties that may not be achievable in real-world op-amps. In this case, we are asked to identify the property that is not a characteristic of an ideal op-amp.

Explanation:
An ideal op-amp is an electronic device that has the following properties:

1. Infinite open-loop gain: This means that the op-amp has an extremely high voltage gain, which is essentially infinite. It amplifies the input signal to a very large extent.

2. Zero input impedance: The ideal op-amp has an input impedance that is infinitely high. This means that it draws no current from the source connected to its input terminals, making it appear as an open circuit.

3. Zero output impedance: The ideal op-amp has an output impedance that is zero. This means that it can drive any load connected to its output terminals without affecting the output voltage.

4. Infinite bandwidth: The ideal op-amp has an infinite bandwidth, which means it can amplify signals of any frequency without any loss or distortion.

5. Zero common-mode gain or infinite common-mode rejection: The ideal op-amp has no response to signals that are common to both its input terminals. In other words, it rejects common-mode signals completely.

Identifying the correct answer:
Out of the given options, the property that is not a characteristic of an ideal op-amp is:
Option A: Zero input impedance

Explanation:
The ideal op-amp assumes zero input impedance, meaning that it draws no current from the source connected to its input terminals. However, in reality, op-amps have non-zero input impedance. This impedance is usually very high, but not infinite. The input impedance of a real op-amp can range from a few kilohms to several megohms. Therefore, option A is the correct answer.

Conclusion:
An ideal op-amp is a theoretical model with certain idealized properties. While it is a useful tool for analysis and design, real-world op-amps have limitations that deviate from the ideal model. In this case, the property that is not a characteristic of an ideal op-amp is zero input impedance.