Answer:
Unsanitary
Step-by-step explanation:
The correct word that will replace the underlined word to improve the overall writing quality of the sentence without changing the overall meaning is unsanitary.
Looking at the word, unsanitary fits in improving the overall writing quality.
Imputing it into the sentence, we will have:
The unsanitary conditions in food processing plants are often responsible for widespread outbreaks of E. coli.
Unsanitary simply means unhealthy, unhygienic, dirty, unclean, impure, contaminated, unwholesome, harmful.
Therefore, unsanitary is the correct word.