Decide which element probably forms a compound with oxygen that has a chemical formula most and least similar to the chemical formula of the compound formed by oxygen and magnesium. Comparing chemical formula of compound formed with oxygen:
strontium potassium selenium lithium
most similar to magnesium
least similar to magnesium

most similar o magnesium : strontium

least similar to magnesium : selenium

Explanation:

For formation of a neutral ionic compound, the charges on cation and anion must be balanced. The cation is formed by loss of electrons by metals and anions are formed by gain of electrons by non metals.  

Magnesium forms a cation having an oxidation state of +2 called as and oxygen forms an anion with oxidation state of -2. Thus they combine and their oxidation states are exchanged and written in simplest whole number ratios to give neutral

Thus an element which will form cation with same oxidation state of +2 will be strontium and form .

Lithium and potassium both belong to group 1 and thus will form cations with  oxidation state of +1 and form and respectivley.

Selenium is a non metal and thus will form covalent compound with formula

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

i believe it is-

d.) gravity pulled particles of dust and gas together to form planets.