In argument, "over half of all automobiles built by the company since 1970 are still on the road today." is trying to play with Simple number percentage flaw.
Here is my line of reasoning for option "B", Please correct me, if, something is wrong in that.
B)- The number of automobiles built by Deluxe each year has not increased sharply since 1970.
You can understand this option by thinking in this way-"Let's say that number of vehicle build were increased. In 1970s they were building 100 vehicle, in 1980s 200 and so on and now in 2020s, suddenly, they are making 10+ million. In this case, even if all vehicle were scraped in same year they were purchased before 2020s, still there are 10+ million vehicle that are still on the road that is far more than half of total build till date. Hence, we can not conclude this is due to durability."
Hope this helps!!!!