The correct answer is option 'D' - Target Controller.

Explanation:
In a data transfer operation involving a SCSI (Small Computer System Interface) BUS, the control is with the Target Controller. Let's understand the roles of the different components involved in a SCSI BUS:

1. Initiator: The initiator is the device that initiates a data transfer operation. It can be a host computer or a device like a disk drive or scanner. The initiator sends commands to the target device and controls the data transfer process.

2. Target: The target is the device that receives commands from the initiator and performs the requested operations. It can be a storage device like a hard disk or a tape drive. The target device responds to the commands sent by the initiator and transfers data as requested.

3. SCSI Controller: The SCSI controller is a hardware component that manages the communication between the initiator and the target devices. It acts as an interface between the SCSI BUS and the host computer. The SCSI controller interprets the commands sent by the initiator and sends them to the target device. It also handles the data transfer process and ensures reliable communication between the devices.

4. Target Controller: The target controller is responsible for controlling the target device's operations. It receives commands from the SCSI controller and coordinates the data transfer process with the target device. The target controller manages the data buffer, handles data transfer rates, and ensures data integrity during the transfer.

Summary:
In a data transfer operation involving a SCSI BUS, the control is with the Target Controller. The Initiator sends commands to the Target Controller, which coordinates the data transfer process with the target device. The SCSI Controller acts as an interface between the initiator and the target device, while the Target Controller manages the target device's operations.