The high current driving capability of the BiCMOS is due to the presence of a Bipolar Junction Transistor (BJT). BiCMOS stands for Bipolar Complementary Metal-Oxide-Semiconductor, which is a combination of bipolar and CMOS technologies. Let's understand in detail why the correct answer is option 'D'.

1. BiCMOS Technology:
BiCMOS technology combines the advantages of both bipolar and CMOS technologies. It utilizes both NMOS and PMOS transistors, along with bipolar junction transistors. The NMOS and PMOS transistors are used for digital logic circuits, while the bipolar transistors enhance the analog capabilities of the circuit.

2. Bipolar Junction Transistor (BJT):
The BJT is a three-layer device consisting of two PN junctions. It has three terminals, namely the emitter (E), base (B), and collector (C). The BJT can operate in two modes: the active mode and the saturation mode.

3. Saturation Mode:
In the saturation mode, the BJT operates as an ideal switch with a low voltage drop across it. The transistor is fully turned on, and it allows a large amount of current to flow from the collector to the emitter.

4. High Current Driving Capability:
The BiCMOS technology utilizes BJTs in saturation mode to achieve high current driving capability. Here's how it works:

- When a large current needs to be driven, the NMOS and PMOS transistors alone may not be able to handle it. In such cases, the BJT is used to provide the additional current driving capability.
- The BJT is connected in parallel with the NMOS and PMOS transistors, acting as a current booster. It allows a high current to flow through it, while the NMOS and PMOS transistors provide the required voltage levels.
- The BJT operates in saturation mode, ensuring a low voltage drop across it and allowing a significant current to flow.
- This combination of the NMOS, PMOS, and BJT allows BiCMOS circuits to drive high currents, making them suitable for power applications and high-speed digital circuits.

In conclusion, the high current driving capability of the BiCMOS is due to the presence of a Bipolar Junction Transistor (BJT) operating in saturation mode. The BJT acts as a current booster, allowing a large amount of current to flow while maintaining low voltage drop.