A constitutional government by definition is a

1. A

   government by legislature

   Government by legislature. Parliamentary, not the definition of constitutional.

2. B

   popular government

   Popular government. Democracy, not limited government.

3. C

   multi-party government

   Multi-party government. A party-system fact.

4. D

   limited government

   Limited government. Public power bounded by a constitution.

Summary. Official key is (d). A constitutional government is limited government — power under, and limited by, a constitution. Legislature, popularity and multi-party cabinets are other ideas.