Thanks a lot.
But the other two will dissolve.
If your spec requires you to know the solubility rules, I suggest you learn them.
I also recommend that you reply to people's posts. That way, they can see that you have posted as there will be an alert.
A: Copper(II) Oxide
What we're doing here is basically adding HCl to the copper oxide, so the reaction would be:
CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)
As you can see, water is formed. Now if you keep on adding CuO until all the HCl has reacted you'll be left with copper chloride dissolved in water right? We know that CuO is insoluble in water so if you keep adding it, it won't dissolve and so you can filter it out. So Copper(II) oxide can't be the answer.
Magnesium reacts with HCl too, so the reaction would be:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Now it's important that you understand what aqueous means. The HCl is aq and so that means there is water present in which the HCl is dissolved in. After the reaction, you'll get MgCl2 dissolved in a bunch of water, hence the (aq). Keep adding magnesium until all the HCl has reacted, you'll end up with MgCl2 in water (hydrogen gas has escaped). Magnesium is insoluble in water and so you can filter it out. So Magnesium isn't the correct answer either.
C: Sodium Hydroxide
Sodium hydroxide reacts with HCl, so again we'll have this reaction:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
In this reaction, it's clear that water has been formed. NaCl is soluble in water and so it will dissolve. After all the HCl reacted, you'll basically have just NaCl dissolved in water. Keep adding NaOH and it WILL dissolve too as NaOH is soluble in water. Therefore, you can't filter out excess sodium hydroxide and will have to use another method in order to get it out. Let's look at our last option in order to be sure.
D: Zinc Hydroxide
Again Zinc Hydroxide reacts with HCl, so the reaction is:
Zn(OH)2(s) + 2HCl(aq) → ZnCl2(aq) + 2H2O(l)
Water is formed in this reaction. The zinc chloride formed is aqueous, meaning it's soluble in water and so it'll dissolve. After all the HCl has been used, you'll end up with ZnCl2 dissolved in some water. Keep on adding Zinc Hydroxide and it won't dissolve, as Zn(OH)2 is insoluble in water. That means you'll be able to use filtration in order to separate from the solution, making it incorrect as well.
So from all of these reactions, the only substance which seems to still dissolve as you keep adding it is the Sodium Hydroxide, making filtration not a possible option for separation.
Therefore, the correct answer is C.