Of course, exactly what constitutes 'the evidence' is almost invariably one of the issues under discussion among the historians who are most deeply engaged with the problem, but in general for each historical question there will be a body of evidence which is recognised as being relevant to it.
This body of evidence will typically comprise what the primary sources tell us about the events and phenomena under discussion.
You will never be asked to produce a narrative of what happened.
In rare circumstances, a few sentences of narrative may form part of the evidence cited in support of a point, but the essay as a whole should be organised according to a logical structure in which each paragraph functions as a premise in the argument.
The evidence almost always permits a variety of solutions, and different approaches generate divergent conclusions.
There are, however, limits to the field of possible solutions, since they must fit in with 'the evidence'.
A good answer will need to harmonise with all of this evidence, or explain why particular items have been dismissed as having no bearing on the problem.
It follows from all of this that — that is, answers which fall outside the field of possible solutions or which fail to take account of received evidence — even though there is no 'absolutely right' answer.
'To-what-extent' questions involve a judgement of measure.
One way of answering the question would be set up a series of 'tests', as it were, that can be investigated in turn.