"child"

in relation to any member, means an unmarried child under the age of eighteen years—

(a)	who is a child for whose maintenance such member is legally responsible;

(b)	who is a stepchild of such member;

(c)	who has been legally adopted by such member;

(d)	any such unmarried child over the age of eighteen years who is a full-time student at any university or other educational institution or who, on account of any physical or mental disability, is not able to provide for his own maintenance and is regularly maintained by such member;