The term ______ means, for the purposes of section 9, any impoundment structure that completely spans a navigable water of the United States and that may obstruct interstate waterborne commerce.

Here is the answer to your question:

Question 75:
The definition provided in the question, "any impoundment structure that completely spans a navigable water of the United States and that may obstruct interstate waterborne commerce," is the definition of an "obstruction" as found in 33 CFR 321.2, specifically for the purposes of Section 9 of the Rivers and Harbors Act of 1899.

• d) obstruction