Magnesium combined with chlorine.
How?
- Look at the question .Compound has -1 charge in it .Hence it contains Halogen.
- Looking at the options Option B and C contain Iodine and chlorine i.e halogens .
Now come to metals side.
- Potassium has valency 1(Look the Electronic configuration below)

Hence it will contain +1 charge which is omitted directly .
Now come to option C.
- Magnesium has charge +2(Look at EC below

Hence the compound formed is given by

Hence option C is correct.
Ow many grams of NH3 can be produced from the reaction of 28g of N2 and 25g
When Newlands tried to create a periodic table, his tried to conform to the "Rule of Octaves" he had discovered. He had the right idea, in that if you arrange the elements by atomic weight there would be similarities every 7 elements (not 8 because noble gases hadn't been discovered yet) but he tried to push this rule so much that he would put multiple elements in the same box to try to keep the rule. Mendeleev, however, left gaps in this table for undiscovered elements, which paved the way for our modern periodic table.
The chemical formula of oxalic is 
When oxalic acid reacts with water, first, oxalic acid removes one proton and results in the formation of mono acids.
After that, in second step, oxalic acid in aqueous solution removes another proton which shows it is a polyprotic acid.
The chemical equations are: (the reactions occurs in two steps due to presence to hydrogen atoms).
When one proton is removed:

When another proton is removed:

The dissociation of oxalic acid in water in shown in the image.