The longest reigning monarch in Thailand is His Majesty King Bhumibol Adulyadej.
The reigning monarch was James the first
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Queen Victoria was the reigning monarch of UK in 1900, until she died the following year.
No, The reigning monarch can not choose the next monarch.
Queen Elizabeth II.
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She is the longest reigning British Monarch ever having recently surpassed Queen Victoria's 64 years on the throne.
That would be a King or Queen.
It was Queen Victoria
The eldest son of the reigning monarch becomes king when the monarch dies. If the reigning monarch has no sons, then his eldest daughter becomes queen. However, the law has just been changed so that the eldest child of the reigning monarch becomes the new monarch on the death of the parent regardless of whether it is a boy or a girl. In practice, as Prince William's wife has just given birth to a boy, it will be at least 80 to 100 years before there is another female monarch.
When the previous reigning monarch dies, at that instant, the next in line becomes the new monarch - this is the accession. At some point in the future (after the old monarch has been ceremonially buried and after a period of morning), the new monarch, at a ceremony, makes promises to their subjects to rule justly and is crowned. At the the point of coronation the monarch becomes the new reigning monarch.