1. el mercado
Answer 1: el mercado moderno
2. los mercados
Answer 2: los mercados modernos
3. la casa
Answer 3: la casa moderna
4. las casas
Answer 4: las casas modernas
Explanation: in Spanish, adjectives must agree with the noun they describe in gender and in number.Adjectives have to agree in gender and number with the noun they accompany.