Answer: Option B. -> base
:
B

Sodium hydroxide, caustic soda - NaOH, is a base because it can render hydroxide ions in aqueous solution. It turns red litmus solution blue.

Answer: Option C. -> Both statements are true
:
C
The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively. Acid rain can cause damage to buildings, historical monuments because they react with the upper layers of the buildings or monuments and corrodes them, they also damage plants and animals by damaging the upper layer of their skin.

Answer: Option A. -> $NaOH$
:
A and D

An acid reacts with a base to neutralize its effect. Hydrochloric acid will react with a base to form salt and water respectively. Here, Sodium hydroxide  and calcium hydroxide are bases so HCl will react with both to form salt and water.

HCl + NaOH $\to$ NaCl + H₂O
$Ca(OH{)}_2$ + $2HCl$ $\to$ $CaC{l}_2$ + $2H_{2}O$

Sodium chloride and water are formed.

Two acids do not react so HCl will not react with sulphuric acid and nitric acid. 

Answer: Option B. -> False
:
B
Phenolphthalein is an acid-base indicator which is colourless in acid solution but turns pink to red as the solution becomes alkaline. 

Answer: Option B. -> Base
:
B
When an ant bites, it injects formic acid in our body, so we should use a base such as sodium bicarbonate. It will neutralize the effect of the acid.

Answer: Option A. -> fertilisers
:
A
The chemical formula of ammonium hydroxide is ${\text{NH}}_4\text{OH}$. It gives ${\text{OH}}^{-}$ ions when added in water. Hence it a base. It is mainly used in the manufacturing of fertilisers.