Step 1: Understand the meaning of "ostentatious."
"Ostentatious" means showy or designed to impress others through display. This suggests something excessive and flamboyant.
Step 2: Analyze the options.
"Lavish" means extravagant or abundant, which conveys a sense of excessiveness, making it a synonym for "ostentatious."
"Simple" and "Modest" are antonyms of "ostentatious," as they refer to things that are understated.
"Unassuming" also implies simplicity and lack of display, making it the opposite of "ostentatious."
Step 3: Conclusion.
The word "lavish" best matches the meaning of "ostentatious." Thus, the correct answer is: \[ \boxed{{Lavish}}. \]