"My grandfather gets tired fast."
Translation:Bunicul meu obosește rapid.
on multi choice Dl also calls repede correct - as bunicul is a male grandparent and as rapid is an adverb not an adjective my instinct is only to allow rapid. Am i wrong? Why would repede be allowed?
Both repede and rapid can be either adjectives or adverbs, depending on context.
In the above sentence, both of them will function as adverbs. Which by the way means that you don't care about the number/gender of the subject.
I would consider "get tired" = "se oboseste" is better, or at least acceptable. Am I correct?
Nope. If you use the reflexive "a se obosi", then it means to take the trouble to do something, to bother with something. It's usually used in the negative, for example:
El nu s-a obosit să o sune ziua următoare.
(He didn't bother to call her the next day.)