Hello there!
Given your question, I can tell you what to look for and what to avoid.
A sentence, is a set of words that complete in itself. They usually have subject, predicate or they convey a subject, question, etc.
A sentence fragment, is a group of words that look like a sentence, but aren't, they are incomplete thoughts.
To be a sentence, the group of words have to contain an Independant Clause.
Which is a group of words that contain both a subject and a verb that can stand on thier own.
So, let's look at the problem in your question.
"The ground was slippery."
In this group of words, "ground," can be marked as the subject matter. The word, "was," is time influenced, so we now know it is past tense. Last but not least, "slippery," is an adjective, a word used to describe a noun.
Are all these things found in a sentence, or a fragment?
You have an idea of what and where it was(remember the word ground) on the subject matter. You have an word that suggests past tense (remember was) and you have a word describing how it was/is (slippery).
Therefore; your answer is- Sentence.
I hope this helps explain it a little bit for future questions related!
Have a good day :)
