The correct answer is option 1, "1 and 2 only". The deficiency of calcium and phosphorus is responsible for weakening of bones.

Calcium is an essential mineral that plays a crucial role in bone health. It is a major component of bones and teeth, and it helps in maintaining their strength and structure. A deficiency of calcium can lead to a condition called osteoporosis, which causes the bones to become weak and brittle.

Phosphorus is also important for bone health. It works together with calcium to provide structural support to the bones. A deficiency of phosphorus can impair the body`s ability to absorb and utilize calcium, leading to weak bones.

Nitrogen and carbon are not directly involved in the strength and structure of bones. Nitrogen is an essential component of proteins, while carbon is a basic building block of organic compounds. Their deficiency does not contribute to bone weakening.

In summary, the deficiency of calcium and phosphorus is responsible for weakening of bones. It is important to ensure an adequate intake of these elements to maintain good bone health.