what his reason__being late again(介词填空)

what his reason__being late again(介词填空)
what his reason__being late again(介词填空)

what his reason__being late again(介词填空)
what is the reason for one's being late again

for

填介词for , the reason for doing sth. 某事的原因
ps:你的问题这句话不完整，要么是你打少了，应该是what is his reason ____ being late again.吧，亦或有下文？