In which kind of reference work would you be most likely to find information about solar energy?

thesaurus

almanac

atlas

encyclopedia

The correct answer is encyclopedia.

A thesaurus is a book where you can find synonyms (words with the same meaning). An almanac is a text which is published yearly, and contains information about various things (usually the weather and crops). An atlas is a book where you can find information about the Earth and its countries. So the only option left is an encyclopedia - a book that contains information about all sorts of things.