Final answer:
A writer can make the sentence less formal by ending it with a preposition, revising it to: "Mr. Liu is shopping for the perfect shelf to display his bowling trophy on."
Step-by-step explanation:
To revise the sentence in a way that avoids overly formal language, a writer could choose to end the sentence with a preposition. The original sentence states, "Mr. Liu is shopping for the perfect shelf on which to display his bowling trophy." A less formal revision could be, "Mr. Liu is shopping for the perfect shelf to display his bowling trophy on." This change maintains the sentence in active voice while simplifying the language and making it sound more conversational.