Explanation for why nickel turns green in hydrochloric acid
The question is really badly worded. For starters, let’s look at solutions of nickel(II):


Figure 1: Nickel(II) solutions. From left to right: $\ce{[Ni(NH3)6]^2+}$, $\ce{[Ni(en)3]^2+}$, $\ce{[NiCl4]^2-}$, $\ce{[Ni(H2O)6]^2+}$. Image taken from Wikipedia, where a full list of authors is available.

You can ignore the left two but the rightmost is a standard nickel(II) solution (could be $\ce{NiSO4}$) while the second from the right is a chlorido-complex—basically $\ce{NiCl2}$ but dissolved in hydrochloric acid. I wouldn’t call the chloride solution ‘yellow’, more like ‘yellowish-green’ but your mileage may vary. Next, let’s look at solids.


Figure 2: salts of nickel(II) chloride. Left or top: hexahydrate. Right or bottom: anhydrous. Images taken from Wikipedia (hexahydrate, anhydrous), where a full list of authors is available.

The picture on the right is dry, i.e. completely water-free nickel(II) chloride (anhydrous). On the left, the salt contains tightly bound water molecules which can be removed by further drying although the salt itself would be dry in everyday terms; this is the hexahydrate of nickel(II) chloride.

It might seem easy that the salt on the left gives a greenish solution while the salt on the right gives a yellowish one. Here’s the catch: they should both give a greenish solution, because there is no excess of chloride as was required in the solution in figure 1.

But what about the actual question? Well, dissolving nickel in hydrochloric acid means that there should in fact be excess chloride, meaning that yellow (or yellowish green) would be the correct answer.

tl;dr: That question is a perfect opportunity to complain if it was marked as ‘incorrect’.