It is called a factual question. Because the answer is a fact rather than a subjective opinion.
Concerned with what is actually the case rather than interpretations of or reactions to it: a mixture of comment and factual information
An explanation of factual question from the book "Spoken Language Understanding: Systems for Extracting Semantic Information from Speech" By Gokhan Tur, Renato De Mori:
The simplest and most frequent question type is a factual question. These questions are questions for which the answer can be a single word or a multi-word expression, often is a named entity. An example question is: Who is the French president?
Another explanation of factual question from the book "Principles of Research Design in the Social Sciences" By Frank Bechhofer, Lindsay Paterson:
When discussing the design of individual questions, it is quite useful to have a basic distinction in our mind between factual questions and opinion questions. An example of a factual question is:
How much wine did you consume last week?
By 'factual' is meant not whether the answer is factual - is true - but rather whether there is in principle a true answer. So the example does have a single true answer, however intrinsically difficult it might to find out what that is.
Lastly, the below excerpt explains the difference between factual and nonfactual question (from the book "Applied Survey Methods: A Statistical Perspective" By Jelke Bethlehem):
Kalton and Schuman (1982) distinguish factual and nonfactual questions.
Factual questions are asked to obtain information about facts or behavior. There is always an individual true value.
The fact to be measured by a factual question must be precisely defined. It has been shown that even a small difference in the question text may lead to a substantially different answer.
Nonfactual questions ask about attitudes or opinions. With opinion and attitudes, there is no such thing as a true value.
There is also a question type called answered question which is considered to have a definite answer. [It is mentioned under philosophic questioning on http://www.uri.edu/personal/szunjic/philos/whystudy.htm]
"Those questions that are already capable of definite answers are placed in the sciences, while those only to which, at present, no definite answers can be given, remain to form the residue which is called philosophy." Thus philosophic questions can turn into scientific truths as soon as they are answered. In other words, many scientifically established truths have started as philosophic questions, but once they received definite answers they get moved to the realm of science.