I am pretty sure the answer is D
The escalation of the Vietnam War and the Watergate scandal led to distrust among the public of the American government. Up until the 1960's and 1970's, Americans were typically approving of the government. They patriotically followed President Roosevelt into World War II and praised the government for helping them out of the Great Depression. The government played it relatively safe in the '50's and Americans didn't mind because they were experiencing great prosperity and didn't want to get involved in another major war. By the time the '60's and '70's rolled around, the Vietnam War was a source of contention because people didn't really agree with the war and the government began hiding information and lying to the public to maintain what little support they still had. The real hit came when Nixon became implicated in the Watergate scandal causing the public to mistrust the government even more. Both of these events demonstrated to the American public that the great leaders of America aren't as great as they're played up to be.
Answer:
Failure at Gallipoli meant that the Allied powers where unable to knock the ottomans out of the war and close another front. This likely would have brought the Central powers to the negotiating table much sooner than it actually happened.
Interesting (unrelated) point: Winston Churchill spearheaded the invasion of Gallipoli
