Which element would mostly have chemical properties similar to that of magnesium (mg)?

The elements of same group are placed in same group as they have similar properties. So Calcium is the element which resembles Mangesium.

Calcium and Magnesium both are alkaline earth metals.

They have similar properties as they have similar outer or valence shell electronic configuration

The general electronic configuration of alkaline earht metal is ns2

The electronic configuration of Mg [atomic number 12] and Ca [atomic number 20] are

 and respectively.

1. a)c-14 has two more protons

2. d)they have different numbers of protons, neutrons, and valence electrons; they are all gases at room temperature.

3. c) iron and nickel.

a corroding iron, if that's one of your choices