Unconditional branch instructions are instructions in a computer program that allow the program to jump to a different location in the code unconditionally, regardless of any conditions or flags. These instructions are used for control flow and can be used to implement loops, conditional statements, and subroutine calls.

Intrasegment direct addressing mode refers to an addressing mode in which the address of the target location is directly specified in the instruction itself. In this mode, the program counter is loaded with the specified address, causing the program execution to jump to that location.

Intrasegment indirect addressing mode refers to an addressing mode in which the address of the target location is specified indirectly through a register or memory location. The program counter is loaded with the value stored in the specified register or memory location, causing the program execution to jump to that location.

Intersegment direct addressing mode refers to an addressing mode in which the address of the target location is specified directly, but the target location is in a different segment or code segment. This is typically used in segmented memory architectures, where different segments of memory are used for different purposes.

In the context of the given question, the addressing mode that is used in unconditional branch instructions is intrasegment indirect addressing mode (option B). This means that the address of the target location is specified indirectly through a register or memory location. The program counter is loaded with the value stored in the specified register or memory location, causing the program execution to jump to that location.

This addressing mode is commonly used in programming languages and assembly languages to implement subroutine calls and jumps to different parts of the code. It allows for flexible and dynamic control flow, as the target location can be determined at runtime based on the value stored in the register or memory location.

In summary, the intrasegment indirect addressing mode is the addressing mode used in unconditional branch instructions, allowing for dynamic control flow in a program.