Answer:
Este vestido es bonito.
Explanation:
Este vestido es bonito. This means: This dress is pretty. In Spanish demonstrative pronouns have gender and can be singular or plural. In this case, the pronoun is modifying "vestido" which is a masculine singular noun. So the pronoun must be male and singular as well. The only possible option is "este". The others: "esta", "esas" and "ésta" are all female so they don't apply.
