A. A wild animal was spotted on Mon March 15
This sentence could do with a comma in between Mon and Mar.
B. A wild wild animal was spotted on Mon., Mar.
This sentence doesn't make sense with ., after Mon
C. A wild animal was spotted on Mon. Mar. 15.
This sentence shouldn't have full stops after Mon, Mar, or 15
D. A wild animal was spotted on Mon, Mar 15.
This sentence makes sense as the comma separates the date.
D is the correct answer.