Arghhh!!!!
You asked:
Which force requires the glider to descend and prevents sustained horizontal flight?
Hint - Gravity is not the answer!!
That is awful English. I don't know why you used the word 'requires' - I suspect you haven't really asked the question you are trying to answer, which is what force or forces control the speed at which a glider descends.
If the word 'makes' was substitued for 'requires' (which is how I read it) the only possible answer to 'what makes the glider descend' is gravity.
However, the glider's ability to resist that force is directly dependent on drag.
Sheer utter b
s. It is mitigated by the
total of
all the upward component of the aerodynamic forces operating on the glider, conventionally described as lift and drag. Or do you think that that the upward component of the lift in the diagram in the URL I provided is a different kind of lift that cannot offset gracity?