Answer:
The sentence that shows the best placement for the modifier "that was on sale" is the following one: Sheila bought soap that was on sale at the farmer's market.
Step-by-step explanation:
A relative clause is a subordinate clause which specifies or gives information about a person or thing. In this case, the relative clause, that was on sale, gives further information regarding the noun soap. What is more, this relative clause is considered essential because it provides necessary information with respect to the noun it refers. If it was dropped, the sentence would not have the same meaning (see 1).
1) Sheila bought soap at the farmer's market
Sheila bought a particular soap, the soap which was on sale, not any soap. With the information included in the relative clause, the reader can identify which soap Sheila bought.