Option 1 -> cautious means careful and wary, which doesn't relate to halfhearted or sporadic efforts.
Option 2 -> impetuous means acting hastily without thought, which is opposite to the lackadaisical nature of desultory.
Option 3 -> guileful means crafty or deceitful, which has no connection to the manner of effort described.
Option 4 -> unenthusiastic means lacking enthusiasm or interest, which aligns with desultory.
Hence, Option 4: unenthusiastic -> "Desultory" means lacking plan, purpose, or enthusiasm; characterized by halfhearted, sporadic efforts that jump from one thing to another without focus. In this context, it describes studying done without genuine commitment or enthusiasm, making it clear the person wasn't serious about succeeding -> correct