Fluoride deficiency in drinking water causes diseases. This deficiency is caused by _______.
a) Water pollution
b) Air pollution
c) Land pollution
d) All of the above

Hint:Land pollution is the contamination of soil by the chemicals, other human or natural activities. Air pollution is the contamination of natural air by human activities majorly, and other climatic changes. The adverse effect of pollution can be seen on the health of plants, animals and human beings as they suffer from different diseases.

Complete answer:
In the human body fluoride (${F^ - }$) ion is contained in the bones and teeth. These are very important parts of human life.
The deficiency of fluoride (${F^ - }$) ion can cause tooth decay and osteoporosis.
Osteoporosis is a kind of disease where the bone weakens, their density decreases and they are very prone to fractures.
As we can see the main component to consume fluoride is drinking water, so improper supply of water will lead to this deficiency.
Hence it is caused by water pollution.
Water pollution can be termed as the contamination of water by human activities. In general because of water pollution the life of aquatic, marine ecosystems gets disturbed. When the water from industries, etc. directly goes into the water body the normal temperature, ions, microorganisms gets disturbed. Hence the dissolved oxygen in water also gets reduced which causes aquatic life including fishes, bacteria, other microorganisms to die.
Thus an entire ecosystem became affected by carelessness.
When water goes for purification the chemical treatments which are performed to remove unwanted substances can also cause the removal of fluorides. So the water treatment plants must have a check on the quality of water. In Australia, the main content of fluorine ions is drinking water.

Therefore, the correct option is (a).

Note:
Fluoride (${F^ - }$) ion deficiency can be treated by drinking good quality water, applying good quality toothpaste as they will cure our tooth, using branded mouthwash, eating foods like oysters, crabs, potatoes, and also should consult a doctor.