Answer :
(a) displacement reaction
(b) combination reaction
(c) disproportionation reaction
(d) displacement reaction
Explanation :
(a) The given balanced chemical reaction is,

This reaction is a single replacement reaction or displacement in which the the more reactive element (Fe) replace the less reactive element (H).
(b) The given balanced chemical reaction is,

This reaction is a combination reaction in which the two reactants molecule combine to form a large molecule or single product.
(c) The given balanced chemical reaction is,

This reaction is a disproportionation reaction in which the chemical species gets oxidized and reduced simultaneously. It is also considered as a redox reaction.
(d) The given balanced chemical reaction is,

This reaction is a single replacement reaction or displacement in which the the more reactive element (Ag) replace the less reactive element (Pt).
The answer to your question is a
Answer:
The responding variable of this experement is the outcome and that would be that the one in lemon juice responded and the one in water didn't (the other one is the control). Thus the responding varible is the one in lemon juice.
Explanation:
The ionic character of any compound depend on the lattice energy as well as the electronegativity of element present in that compound.
More would be the lattice energy more would be ionic nature of that compound.
The lattice energy of any compound is inversely proportional to the ionic radii cation and anion.
In given case the ionic radii of oxide in both oxides would be equal therefore the lattice energy only depend on the ionic radii of cation.
As the radii of Magnesium less then radii of lithium therefore lattice energy of Magnesium oxide would be more than lithium oxide.
Hence, MgO would be more ionic in nature than 