Which word describes behavior that is improper or inappropriate?

The word "unseemly" is the correct choice because it specifically refers to behavior that is not proper or appropriate according to social standards or expectations. When someone exhibits unseemly conduct, it often implies that their actions are not only inappropriate but also violate accepted norms of decency and decorum. This term captures the essence of behavior that is considered unacceptable or unbecoming in a given context.

In contrast, other options do not focus on the notion of impropriety. "Usurp" pertains to taking a position of power illegally or by force, which is unrelated to behavior norms. "Superficial" describes something that is shallow or lacking in depth, which does not inherently carry a connotation of impropriety. Lastly, "tactful" refers to a person’s ability to say or do the right thing in social situations, promoting considerate and appropriate behavior rather than improper actions.