Final answer:
The correct adjective ending is 'es' to match the neuter noun 'Buch' in German.
Step-by-step explanation:
The correct adjective ending in this case is es. Therefore, the correct phrase would be 'Er liest ein gutes Buch.'
In German, the ending of an adjective depends on the gender, case, and article of the noun it modifies. In this case, 'Buch' is a neuter noun, so the adjective 'gut' takes the ending 'es'.
Here are some more examples of adjective endings in German:
- Der ____________ Tisch (masculine, nominative): gute
- Die ____________ Schule (feminine, nominative): gute
- Die ____________ Bücher (masculine, accusative): guten
Learn more about Adjective endings in German