Unsupported. We know that some overworked students are happy, but we don’t know if most are. Let’s say there are 100 serious students: 51 are happy and 51 go to grad school. One student must be in both groups, but the other 99 could only be in the “happy” or “grad school” group!

Must be true. Over 50% of serious students are happy; over 50% of serious students go to grad school and are therefore overworked. Thus, there must be overlap. Let’s say out of 100 serious students, 51 are happy and 51 go to grad school/are overworked. One student must be both!

Unsupported. We know that most serious students are overworked, but that doesn’t tell us anything about how many overworked students are serious!

Unsupported. While we can’t properly infer from the stimulus that all grad students are happy (although we can infer that some are), we also can’t rule out that possibility.

Unsupported. We can infer that most serious students are overworked, but there’s no evidence in the stimulus to support that all serious students are overworked.