Among the given vitamins :
(a) Source of vitamin $\mathrm{B}_6$ is milk and it causes deficiency disease called 'convulsions'. Thus, it is the correct statement.
(b) Green leafy vegetables are the source of vitamin $\mathrm{K}$ and it is used to increase the blood clotting period. Thus, the given statement is not correct.
(c) Citrus fruits and green leafy-vegetables are the source of vitamin-C and 'scurvy' is the deficiency disease. Thus, the given statement is not correct.
(d) Exposure to sun-light and egg yolk are the source of vitamin-D and 'ricket' is the deficiency-disease. Thus, the given statement is not correct. Hence, option (a) is the correct answer.
