The problem in the italicized element in the sentence below is
the party was very nice.

a. trite expression
b. slang
c. colloquialism/stilted language
d. vague language
!

D: vague language

Explanation:

The problem in the italized element in the sentence is vague language. The adverb very is used to emphazise the the adjective nice. But this word 'nice' is  vague, and often is better to use a word that describes wuat someone wants to express more exactly (more specific adjectives), for example: enjoyable, fun, pleasurable, good.

is this your complete question

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.