The Olympic Games helped to keep peace among the pugnacious states of the Greek
world in that a sacred truce was proclaimed during the festival’s month.
(A) world
in that a sacred truce was proclaimed during the
festival’s monthAs per OG explanation,
“in that” is overly formal and stilted, and therefore, ‘for’ should be preferred over the same.
Also, “festival’s month” is an informal way of saying the idea “the month of the festival”. On GMAT, we should prefer the latter way.
(B) world,
proclaiming a sacred truce during the
festival’s month“proclaiming” is a verb-ing modifier following a clause and preceded by a comma. Therefore, it acts as a clause modifier. However, it doesn’t make sense with the subject of the preceding clause. The Games cannot proclaim anything!
Also, “month of the festival” is preferred over “festival’s month”.
(C) world when
they proclaimed a sacred truce for the festival month
There is no antecedent of “they”. “they” cannot logically refer to ‘Olympic Games’. Also, “month of the festival” is preferred over “festival month”.
(D) world, for a sacred truce was proclaimed during the month of the festival
CORRECT. For is used as BECAUSE over here. Other Examples include,
I was tired,
for I had played cricket throughout the day..
I was very happy with her,
for she supported me in all situations..
I gave few examples on usage of 'for',
for smartyguy wanted clarity on the usage..
(E) world by proclamation of a sacred truce
that was for the month of the festivalLike option B, this option also indicates that the Olympic Games did the proclamation! (Here "By proclamation" refers to the subject of the sentence -- The Olympic Games similar to the VERB-ING modifier in option B)
Also, this option distorts the meaning of the sentence. This option means that
the sacred truce was always for the month of the festival which is NON-SENSICAL (as this information is presented by using a ‘that’ clause modifying ‘sacred truce’). Per the intended meaning it was "during the month of the festival."