Select the most appropriate option to fill in the blank:

The teacher emphasized the importance of discipline, stating that it is the _________ of success.

Option D is the correct answer. Cornerstone refers to something that is essential, fundamental, or the foundation of success. Discipline being emphasized as a key to success aligns perfectly with this metaphor.

Option A: Impediment means an obstacle or hindrance, which is the opposite of what discipline represents in this context.

Option B: Shadow does not convey the idea of a foundational element; it implies something secondary or unimportant.

Option C: While a blueprint represents a plan or guide, it doesn’t fit as well as "cornerstone," which emphasizes discipline as the foundation of success.

Option E: Drawback means a disadvantage, which is contradictory to the positive role of discipline in achieving success.