PC relative mode is especially used for branching.
The branch address specified by the instruction is added to the contents of the PC (Program Counter).
An instruction is stored at location 300 with its address field at location 301.
This simply means that starting address of the instruction is at location 300, and it spans till location 301. ie, One instruction occupies two memory locations. (Some part stored at 300-301, remaining portion stored at 301-302)
What does PC point to?
At first PC must have pointed to 300, that's how it reached this instruction.
Now, as soon as this instruction got fetched, PC started to point at the next instruction.
Where is the next instruction stored? 301? No. Because 301 has some portion of the former instruction.
The next instruction is at 302.
So, PC points to 302.
Now branch address = 400. (given)
So, Effective memory address = 400 + 302 = 702.
R1 contains the number 200. If relative-addressing mode is used by the instruction (with R1 as index register)
This information is irrelevant here. We don't need any index registers in PC relative mode.
Index registers are used in Indexed Mode.