The lack of which vitamin causes rickets?

The most common cause of rickets is a lack of vitamin D or calcium in a child's diet. Both are essential for children to develop strong and healthy bones. Sources of vitamin D are: sunlight – your skin produces vitamin D when it's exposed to the sun, and we get most of our vitamin D this way; food – vitamin D is also found in some foods, such as oily fish, eggs and fortified breakfast cereals; and dietary supplements.

Source: NHS