The answer is: by telling himself that he didn’t study hard enough for the test.
An individual with an internal locus of control would believe that they have control over the events that occur in their lives. This is a concept that was theorized by Julian B. Rotter, together with its opposite, external locus of control. Individuals with external locus of control tend to believe that they have no control over their lives – that life is controlled by outside factors that they cannot do anything about.