$$\textbf{Correct answer: A}$$
$$\textbf{Option (A):}$$
$$\bullet$$ The weakness of the muscles and bones in the elderly is due to the deficiency of Vitamin D.
$$\bullet$$ Vitamin D3 is responsible for the deposition of calcium in the bones and muscles. In the absence of you, there is induced bone density leading to osteoporosis.
$$\bullet$$ This is responsible for increasing absorption of calcium, magnesium and phosphate.
$$\bullet$$ The deficiency of this occurs in the old days leading to higher chances of having fractures and soft or weak bones. It also causes muscle weakness.
$$\textbf{Option (B):}$$
$$\bullet$$ The deficiency of Vitamin C causes scurvy.
$$\textbf{Option (C):}$$
$$\bullet$$ Vitamin B complex deficiency causes diseases like Pernicious anemia, pellagra.
$$\textbf{Option (D):}$$
$$\bullet$$ Vitamin A deficiency is responsible for causing night blindness.
Hence, the correct answer is option (A).