Breakdown of the Blank
The sentence describes Stephen's behavior as a procrastinator: "Stephen found himself unable to recall a time when he did not put off doing a major assignment."

This evidence indicates that Stephen's procrastination is not occasional or random; it is constant, deeply ingrained, and a long-standing habit.

We need two synonyms that mean long-established, habitual, or deep-rooted.
- A. insatiable: Impossible to satisfy (related to desire or appetite). This applies to the desire to procrastinate, not the quality of the habit.
- B. habitual: Done constantly or regularly; established by habit. This is a perfect fit.
- C. derelict: In a very poor condition as a result of neglect. This describes a state of things, not the nature of a person's habit.
- D. erratic: Not even or regular in pattern or movement; unpredictable. This is the opposite of a constant, established habit.
- E. inveterate: Having a particular habit, activity, or interest that is long-established and unlikely to change. This is a perfect synonym for "habitual" and fits the context of being unable to recall a time when he didn't do it.
- F. untiring: Continuing at length without losing energy or enthusiasm. This describes energy, not the quality of the habit.

The two words that mean long-established or regular are habitual and inveterate.
The correct choices are B. habitual and E. inveterate.