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Answer:- b. +3
Explanations:- Charge of a metal ion is equal to the number of valence electrons it has as it loses these electrons to have its nearest noble gas like electron configuration.
Al has 3 valence electrons and being a metal it loses all of these and so the charge of Al ion is +3. It could be shown as:

So, the right choice is b. 3+.
Answer:
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I hope this helps!
You have to put your attention to the unit of concentration. It is expressed in terms of molarity, which is represented in M. It is the number of moles solute per liter solution. So, you simply have to multiply the molarity with the volume in liters.
Volume = 275 mL * 1 L/1000 mL = 0.275 L
<em>Moles Ba(OH)₂ = (0.200 M)(0.275 L) = 0.055 mol</em>