The third sentence is the correct answer - <span>When the Spaniards decided to conquer Mexico, they appointed Hernan Cortes to lead the army.
Given that there is an inversion in this sentence (the dependent clause is placed before the independent one), comma has to be placed after the dependent clause, and before the independent one. So, the comma should be after <em>when the Spaniards decided to conquer Mexico.</em>
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I believe it would be testimony.
In favour, if i understand the question correctly