For drinking purpose, water should be tested for
A) Fluoride
B) Lead
C) Sulphate
D) Nitrate

Hint: For drinking purpose water should be checked as it contains many minerals and ions which makes water hard. Before drinking, TDS which is the total dissolved salt of the water must be checked.

Complete step by step answer:
For drinking purposes, water should be checked as it comes from different sources and may contain poisonous metals like mercury or lead.
A) Fluoride: It is one of the important elements in the water as it prevents tooth decay but if it is consumed slightly more than normal for a long time then it is as toxic as lead. It may cause Fluorosis, osteoporosis, arthritis, and even cancer. So, the safe concentration of fluoride is $1.5mg/l$. below this concentration, it may lead to tooth decay.
B) Lead: It is also present in water from the sources like industries, Hospitals, etc. and its safe concentration in water is $0.01mg/l$. Above this concentration, it may cause several problems in humans like Kidney problems, infertility in both men and women, and hence its concentration must be regulated before sending them for drinking purposes.
C) Sulphate: Its safe concentration is $250mg/ml$ and Its excess concentration is not much harmful to the human.
D) Nitrate: It is safe for drinking up to a concentration of $10mg/ml$. It is one of the most common groundwater contaminants and its excess amount causes ‘Blue baby syndrome ’ or ‘methemoglobinemia’. So before drinking, the concentration of all the minerals above should be checked.
So, the correct option is (A), (B), (C) and (D).

Additional information:
Before drinking, TDS (Total dissolved salt) of the water must be checked. It measures the amount of mineral, salt, and metal present in water. TDS less than $300mg/l$ is ideal for drinking more than $1200mg/ml$ is highly unacceptable.

Note: There are two more very toxic elements for the human, mercury, and cadmium. They accumulated in the human through the process of biomagnification. Due to mercury toxicity, Minamata disease caused in Japan and toxicity of Cadmium causes Itai-Itai disease.