WiMAX, which stands for Worldwide Interoperability for Microwave Access, is a wireless communication technology that provides high-speed internet access over long distances. It is based on the IEEE 802.16 standard and operates in the microwave frequency range.

Orthogonal Frequency Division Multiplexing (OFDM) is the modulation scheme employed by WiMAX, making option A the correct answer. OFDM is a digital modulation technique that divides the available bandwidth into multiple subcarriers, each of which carries a portion of the data. These subcarriers are orthogonal to each other, meaning they do not interfere with one another, allowing for efficient use of the available spectrum.

Below, we will explain in detail why WiMAX uses OFDM and why the other options are not correct:

Orthogonal Frequency Division Multiplexing (OFDM)
- OFDM is a widely used modulation scheme in wireless communication systems, including WiMAX.
- It divides the available bandwidth into multiple subcarriers, each carrying a portion of the data.
- The subcarriers are orthogonal to each other, meaning they can be transmitted simultaneously without interfering with one another.
- This allows for efficient use of the available spectrum, increasing the overall data throughput.

Time Division Multiplexing (TDM)
- TDM is a different modulation scheme that divides the available bandwidth into time slots.
- Each user or device is assigned a specific time slot to transmit their data.
- While TDM is used in some communication systems, WiMAX does not use it as its primary modulation scheme.

Space Division Multiplexing (SDM)
- SDM is a technique that uses multiple antennas to transmit and receive data simultaneously.
- It takes advantage of the spatial dimension to increase the overall data throughput.
- While SDM is used in some wireless communication systems, WiMAX does not rely on it as its primary modulation scheme.

Conclusion
In summary, WiMAX uses Orthogonal Frequency Division Multiplexing (OFDM) as its primary modulation scheme. OFDM allows for efficient use of the available spectrum by dividing it into multiple subcarriers that can be transmitted simultaneously without interference. This increases the overall data throughput and enables high-speed internet access over long distances.