Answer:
C. One important aspect of the book is it detailed descriptions.
Explanation:
A. <em>At the party there were no one I knew.
</em>
Wrong. The subject is in the first person singular form. Thus, the use of the verb "were" should be changed to "was".
B. <em>There was many problems with the construction of the house.
</em>
Wrong. The use of the verb "was" is wrong. The subject is plural, thus the verb will also have to be in the plural form.
C. <em>One important aspect of the book is it detailed descriptions.</em>
Correct. The verb "is" agrees with the subject "it".