Answer:
D. Dion's homecoming party will be on Saturday, September 19, 2015.
Step-by-step explanation:
A. Dion's homecoming party will be on Saturday September 19, 2015.
- (Missing comma after Saturday)
B. Dion's homecoming party will be on Saturday, September 19 2015.
C. Dion's homecoming party will be on Saturday, September, 19, 2015.
- (Comma should be removed after September)