In (a), ‘who scored good marks’ is a clause.
[ subject- who, predicate- scored good marks (verb- scored)]
It qualifies the noun ‘student’. So it is an Adjective Clause.
In (b), ‘which is like a tiger’ is a clause.
[Subject- which, predicate- is like a tiger (verb- is)]
It qualifies the noun ‘dog’. So it is an Adjective Clause.