Yes answer is option A ,
Just wanted to add a point :
Always think of worst case. coz one can argue that "I ll not give any student marks>=50" then no array will be needed but that couldn't be the case ''always".
So worst case would be -> allocate every student marks>=50 then max size of array neede is "50" because by pigeon hole principle , after 100 , elements will be repeated.
P.S :
Dont think like " I will give 51 marks to every student so that array of size only "1" will be sufficient.., bcause this is again not the worst case.