an inert electrode is made up of non reactive metals like platinum
an inert is electrode is required in one of the half cells or both the cells if there is no solid conducting metal in either half reaction. or we can say that if in any half reaction neither product nor reactant is in solid phase.
from the given equations it is clear that
a) in first cell we have cu and ag in solid phase in either side
b) in second reaction the iron is in aqueous phase on both the side of reactant and product
so for reduction of iron or in cathodic half cell we need inert electrode [which does not participate in reaction]
hence answer is
c) in third reaction zn and sn are in solid phase in either side
d) in fourth reaction al and ni are in solid phase in either side
the answer is c because material is worn away because of weathering>
an ionic compound consists of a metal and a non-metal.
oxygen and fluorine are non-metals.
sodium and aluminium are non-metals.
calcium is a metal and chlorine is a non-metal.
nitrogen and sulfur are non-metals.
thus, the answer is c.