Final answer:
Monsanto controlled 30% of the lawn-care treatment market by the late 1980s, largely due to its product Roundup.
Step-by-step explanation:
By the late 1980s, the company that controlled 30% of the lawn-care treatment market was Monsanto. This dominance was largely due to its successful herbicide product, RoundupĀ®, which was discovered by Monsanto chemist John E. Franz in 1970. Roundup gained a reputation for being an effective weed killer and could be applied in various settings, contributing to Monsanto's significant market share in lawn-care treatments.