B 1. magnesium hydroxide + sulfuric acid sulfate +————

2. calcium hydroxide + hydrochloric acid calcium
+ water+————

3. lead hydroxide + nitric acid lead nitrate +————-

4. sodium hydroxide + sulfuric acid
+ water————-

5. potassium hydroxide + citric acid potassium
+ water————-

6. silver hydroxide + hydrochloric acid chloride +———

7. aluminium hydroxide + phosphoric acid———-

 1. magnesium hydroxide + sulfuric acid = magnesium sulphate + water

2. calcium hydroxide + hydrochloric acid = calcium  chloride + water

3. lead hydroxide + nitric acid =  lead nitrate + water

4. sodium hydroxide + sulfuric acid  =  sodium sulphate + water

5. potassium hydroxide + citric acid = potassium  citrate + water

6. silver hydroxide + hydrochloric acid = silver chloride + water

7.  aluminium hydroxide + phosphoric acid = aluminium phosphate + water

Explanation:

A double displacement reaction is one in which exchange of ions take place. The salts which are soluble in water are designated by symbol (aq) and those which are insoluble in water and remain in solid form are represented by (s) after their chemical formulas.  

The balanced chemical equations are :

1. magnesium hydroxide + sulfuric acid = magnesium sulphate + water

2. calcium hydroxide + hydrochloric acid = calcium  chloride + water

3. lead hydroxide + nitric acid =  lead nitrate + water

4. sodium hydroxide + sulfuric acid  =  sodium sulphate + water

5. potassium hydroxide + citric acid = potassium  citrate + water

6. silver hydroxide + hydrochloric acid = silver chloride + water

7.  aluminium hydroxide + phosphoric acid = aluminium phosphate + water

there are two significant figures.