The noun related to "obsolete" will be -

Let's look at the meaning of the word and the marked option. Obsolete (adjective): no longer produced or used; out of date.Obsolescence (noun): the state of being which occurs when an object, service, or practice is no longer wanted even though it may still be in good working order. Example: ​Mobile phone technology is developing so quickly that many customers are concerned about obsolescence. ​Hence from the given meaning and example, we find that the correct answer is: Obsolescence.