39
(A) by means of
(B) in honor of
(C) in order to
(D) in place of

by means of  藉著...方法(或手段)
in honor of  紀念, 向...表敬意
in order to  為了…
in place of  代替; 較喜歡

  Many superstitious beliefs are connected to religion. For instance, if one hold out抵禦, 供應、持續 a cross or wears one, the person can ward off避開；阻止 evil.

Certain talismans護身符 worn（wear 的過去分詞）or hung in houses were to have warded off evil.

Saint Gregory I the Great ordered that people say "God Bless You" when someone sneezed 39 ward off disease. 

39 
(A) by means of中文:用、藉著...方法(或手段)

(B) in honor of中文表示對…的敬意,為慶祝，為紀念

不定詞片語 in order to + V 表達「為了…」

Saint Gregory I the Great ordered that people say "God Bless You" when someone sneezed 39 ward off disease

可以發現Ward off 並沒有變成動名詞，所以選   in order to   

Saint Gregory I the great ordered that people say god bless you  when someone sneezed____ward off （避開）disease.