The correct option is
B he would have scored more marks
The given sentence is a type 3 conditional, as the if-clause is in past perfect tense 'had studied'.
The type 3 conditional has the following structure:
if-clause 'past perfect tense' + main clause 'perfect conditional'
where the if-clause 'if Somu had studied well' expresses an unreal past condition and the main clause expresses its probable past result 'to score more marks'.
Option C: 'He would have scored more marks' is a perfect conditional that agrees with a type 3 structure.
Hence, option C is correct.
Option A: 'He will score more marks' states a future action. But a type 3 conditional states a past condition.
Option B: 'He would score...' is a present conditional, seen in type 2 and not type 3 conditionals.
Option D: 'He had scored...' is in simple past tense and does not express a probable result, but an action that actually happened.
Hence, options A, B and D are wrong.