http://i64.tinypic.com/242imwz.jpg - Question 15 why is the answer (d)..?

15.
A. False
Al³⁺ reacts with aqueous ammonia to form a white precipitate of Al(OH)₃, which does NOT dissolve in excess ammonia.
Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)

B. False
Ca²⁺ does NOT form precipitate with aqueous ammonia.

C. False
Cu²⁺ reacts with aqueous ammonia to form a BLUE precipitate of Cu(OH)₂, which dissolve in excess ammonia.
Cu²⁺ (aq) + 2OH⁻(aq) → Cu(OH)₂(s)
Cu(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄]²⁺(aq) + 2OH⁻(aq)

D. True
Zn²⁺ reacts with aqueous ammonia to form a white precipitate of Zn(OH)₂, which dissolves in excess ammonia.
Zn²⁺ (aq) + 2OH⁻(aq) → Zn(OH)₂(s)
Zn(OH)₂(s) + 4NH₃(aq) → [Zn(NH₃)₄]²⁺(aq) + 2OH⁻(aq)

...... The answer is: D. Zn²⁺