The correct answer is option 'A': TTL logic level.

Explanation:

TTL stands for Transistor-Transistor Logic, which is a type of digital logic circuit family that is widely used in micro-controllers and micro-processors. TTL logic level refers to the voltage levels that are used to represent binary states in these devices.

Here are the reasons why TTL logic level is the logic level understood by micro-controllers/micro-processors:

1. Compatibility: TTL logic level is the most commonly used logic level in micro-controllers and micro-processors because it is compatible with the internal circuitry of these devices. The input and output pins of these devices are designed to work with TTL logic levels.

2. Voltage Levels: TTL logic level uses a supply voltage of 5V, where a logic 0 is represented by a voltage between 0V and 0.8V, and a logic 1 is represented by a voltage between 2V and 5V. These voltage levels are well-suited for the operation of micro-controllers and micro-processors.

3. Noise Immunity: TTL logic level has good noise immunity, which means it can tolerate noise and interference in the signal without affecting the logic state. This is important in the operation of micro-controllers and micro-processors, as they are often used in environments with high levels of electrical noise.

4. Speed: TTL logic level provides fast switching speeds, which is crucial for the efficient operation of micro-controllers and micro-processors. The fast switching speeds allow these devices to process data quickly and execute instructions in a timely manner.

In contrast, RS232 logic level is not directly understood by micro-controllers/micro-processors. RS232 is a serial communication standard that uses different voltage levels (-12V to +12V) and signaling protocol compared to TTL logic level. While micro-controllers/micro-processors can communicate with RS232 devices using level shifters or UART (Universal Asynchronous Receiver-Transmitter) modules, the internal logic level of these devices is still TTL.

In conclusion, the logic level understood by micro-controllers/micro-processors is TTL logic level, as it is compatible, uses appropriate voltage levels, provides noise immunity, and offers fast switching speeds, making it ideal for the operation of these devices.