D. the person has ridden other roller coaster that weren't as good as that one
This option is accurate, since from the sentence, "That was the greatest roller coaster ride of my life!" it can be inferred that the speaker has previously ridden different roller coasters that might have even been good, but as great as the one he or she is currently referring to.
Additionally, the word "greatest" is in superlative degree implying that the speaker thinks the rollercoaster is the best out of all other rollercoasters he or she has previously ridden.