Query about Question 6 of "PrepSwift: # of Numbers in Factorials II" [12371]

GRE GRE Quant Problem Solving
1

https://prepswift.gregmat.com/quizzes/quiz/of-numbers-in-factorials-ii-quiz (Question 6)

Can somebody explain this in simple terms? I fail to understand this solution.

2

Refer to these explanations

Incomplete/poor explanation of Question 6 from "PrepSwift: # of Numbers in Factorials II" [12371] - #2 by Kweku .

Query about Question 6 from "PrepSwift: # of Numbers in Factorials II" [12371] (half the answer) - #3 by Kweku