4. the formula for magnesium hydroxide is\na mgoh\nb mg(oh)₂\nc mg₂oh\nd mg₂(oh)₂

Explanation:

Step1: Determine the charge of magnesium ion

Magnesium ((Mg)) is in Group 2 of the periodic table, so it forms a (Mg^{2 + }) ion.

Step2: Determine the charge of hydroxide ion

Hydroxide ion is (OH^{-}).

Step3: Use the criss - cross rule for ionic compound formula

To balance the charges ((+ 2) from (Mg^{2+}) and (-1) from (OH^{-})), we need 2 (OH^{-}) ions for each (Mg^{2+}) ion. So the formula is (Mg(OH)_{2}).

Answer:

B. (Mg(OH)_{2})