Final answer:
Using recency theory, a firm should advertise lawn mowers on online platforms that are likely to reach customers shortly before they make a purchase decision, as newspapers and magazines are becoming less effective.
Step-by-step explanation:
Based on the recency theory, the best media strategy for a firm advertising lawn mowers would be to prioritize platforms that ensure advertisements are seen by potential customers as close as possible to the time of decision-making or purchase. Given that newspapers and magazines are now less cost-effective compared to the Internet and social media due to technological advancements, the firm should focus on online channels. These channels could include targeted social media campaigns, search engine marketing, and display ads on relevant websites, taking advantage of higher traffic and the ability to reach audiences at critical times with precision.