Final answer:
Shauna was killed on Sunday morning. The police used deductive reasoning and eliminated all other possibilities to determine that Mark was the killer.
Step-by-step explanation:
In this scenario, Shauna was killed one Sunday morning. The police were able to determine who to arrest based on the activities of the other individuals mentioned.
The police concluded that Mark was the killer. The reason for this is that Sunday morning is not included in the list of activities. Since everyone else has an alibi or is accounted for, Mark becomes the prime suspect.
This logic puzzle demonstrates the use of deductive reasoning, where the police eliminate other possibilities and deduce that Mark is the only one who could have committed the crime.