who hold the power to decide this?
Monarchy and constitution, monarchs may hold formal authority and the government may legally operate in the monarch's name
Constitutional monarchy
A constitutional monarchy is a form of monarchy in which the sovereign exercises authority in accordance with a written or unwritten constitution. Constitutional monarchy differs from absolute monarchy (in which a monarch holds absolute power) in that constitutional monarchs are bound to exercise their powers and authorities within the limits prescribed within an established legal framework. Constitutional monarchies range from countries such as Liechtenstein, Monaco, Morocco, Jordan, Kuwait and Bahrain, where the constitution grants substantial discretionary powers to the sovereign, to countries such as the United Kingdom, Spain, Belgium, Sweden and Japan, where the monarch retains significantly less personal discretion in the exercise of their authority.

when was the last time an english monarch vetoed an act of parliament UNANSWERABLE what is this topic about? Royal assent is the method by which a monarch formally approves an act of the legislature. which country does it belong to ? In those nations which still, in theory, permit the Monarch to withhold assent to laws such as the United Kingdom, Norway, and Liechtenstein. how is it decided upon? By modern constitutional monarchy.
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Constitutional monarchy [SEP] Introduction
A constitutional monarchy is a form of monarchy in which the sovereign exercises authority in accordance with a written or unwritten constitution. Constitutional monarchy differs from absolute monarchy (in which a monarch holds absolute power) in that constitutional monarchs are bound to exercise their powers and authorities within the limits prescribed within an established legal framework. Constitutional monarchies range from countries such as Liechtenstein, Monaco, Morocco, Jordan, Kuwait and Bahrain, where the constitution grants substantial discretionary powers to the sovereign, to countries such as the United Kingdom, Spain, Belgium, Sweden and Japan, where the monarch retains significantly less personal discretion in the exercise of their authority.