George Washington's greatest challenge at Valley Forge was keeping his army together. The morale of his army was dwindling and so it was a challenge for Washington to keep the army together. The correct option among all the options given in the question is the third option or option "C".
The correct answer to this open question is the following.
You did not include the text or a link to it, neither any options for this question. However, we can comment on the following general terms.
If you are talking about United States politics at the beginning of the nation, then prevailing views on why parties formed in the early republic were that since the beginning there were two different views of how to form a government for the United States.
One side was known as the Federalists, led by people like Alexander Hamilton, Jhon Jay, and James Madison. They believed in a strong central government for the new nation. On the other hand, there were the Antifederalists like Thomas Jefferson, who opposed a strong central government because they thought it could derive into tyranny, as had been the case of the English monarchy.
Indeed, that is what President George Washington feared and the reason why he did not support the formation of parties, because they could lead to a divided nation.