If he ______ his studies seriously in college, he would have a more successful career today.
Correct: A
This is a Mixed Conditional sentence (Type 3 in the 'if' clause, Type 2 in the main clause). It describes a past hypothetical condition ('if he had taken his studies seriously') leading to a present result ('he would have a better job today'). The past perfect tense is used in the 'if' clause for the past condition.