(B) Both (A) and (R) are true but (R) is not the correct explanation of (A).
The Act of Union 1707 did form the United Kingdom of Great Britain (A is true), and England did impose its influence on Scotland (R is true), but (R) is the result/effect of the Act, not the reason/explanation for it.
The textbook states that the Act of Union resulted in England being able to impose its influence on Scotland — meaning England's dominance over Scotland was a consequence of the Act, not the cause/explanation for it. So both statements are factually correct, but R does not explain why A happened; hence option (B).