All of the following were provisions of the compromise of 1850 except:
a. california was admitted as a free state
b. the fugitive-slave law was strengthened
c. ended the slave trade in the district of columbia
d. the district of columbia was closed to slavery

The answer is D. the district of Columbia was closed to slavery.
Senator Henry Clay introduced a series of resolutions on January 29, 1850, in an attempt to seek a compromise and avert a crisis between North and south. As part of the Compromise of 1850, the Fugitive Slave Act was amended and the slave trade in Washington DC was abolished.