The correct option is C (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)
(a) Be is used in windows of X-ray tubes because of its low mass absorption coefficient.
(b) Magnesium is used in either powdered or solid form as an incendiary agent for both illumination and antipersonnel purposes.
(c) Pure calcium metal is used as a reducing agent in the preparation of other types of metal, such as thorium, uranium and zirconium.
(d) Due to its radioactive nature Ra is used in the treatment of cancer.