Based on the grammar and context, the correct answer is:
"are sliced"
Here's why:
* The sentence describes a present action or a general truth.
* "Tomatoes" is a plural noun, so it requires a plural verb.
* "Are sliced" is the present simple passive form, which is appropriate in this context.
Therefore, the complete sentence is:
"Use a sharp knife! The tomatoes are sliced thinly for this salad."
Answer: are sliced