Which of the following best defines 'obsolete'?

The term 'obsolete' best aligns with the meaning of "out-of-date." It refers to something that is no longer in use or has fallen out of favor due to advancements or changes in technology, preferences, or knowledge. When something is considered obsolete, it indicates that it lacks relevance or utility in the current context, often being replaced by newer alternatives that better meet present needs.

The other choices imply a sense of currency or progress that contradicts the definition of being obsolete. Current and popular suggests a state of being widely accepted and used, which is the opposite of being outdated. Frequently changing denotes a dynamic state of evolution, while highly innovative speaks to cutting-edge advancements, both of which also contrast with the idea of something being out-of-date. Thus, the definition of 'obsolete' as out-of-date accurately captures its essence.