Hello Mushraf,
welcome to Javaranch!
When it comes to Garbage collection, an image is worth a thousand words, if you REALLY want to solve garbage collection problems ONCE-AND-FOR-ALL practice diagram drawing, it is NOT difficult, it took me ONLY 90seconds to solve this problem.
The answer is ONE object is 'eligible' for garbage collection. Which is the object referenced by 'a2.a'
I guess you REMEMBER that objects are 'eligible' for garbage collection when they have NO 'live reference' variables connecting them.
The ONLY way I can effectively explain this to you is via diagram drawing. (I promise to do that later today).
Regards
Ikpefua.