The word "unmitigated" means "not diminished or moderated in intensity or severity; sometimes used as an intensifier".

It can also mean "absolute, unqualified, unconditional, categorical, or complete". In this context, the word "unmitigated" emphasises how bad the speaker's experience at the university was. It implies that there was nothing good or positive about it.

Therefore, the option that is nearest in meaning to "unmitigated" is D. an unqualified disaster. This means a total disaster, without any exceptions or qualifications.

The other options are not as close in meaning to "unmitigated".

An unavoidable disaster means a disaster that could not be prevented or avoided. A painful disaster means a disaster that causes a lot of suffering or distress.

A regrettable disaster means a disaster that causes sorrow or remorse. An unfortunate disaster means a disaster that is unlucky or unfavourable.