A. a mixture is a combination of two or more substances in which each substance retains its own properties.
Option (a) is correct.
A reducing agent is the one which loses electrons to other substance.
Here, Zn has oxidation number 0 in the L.H.S of the equation, but on R.H.S its oxidation number is +2 i.e. it Zn has donated two of its electrons to

.
Hence, Zn is the reducing agent here.
True, it would slow down.
Not sure, is it <span>:
</span>B.
<span> 1.9 3 1027 NiCl2
formula units</span>