You are trying to load data from Azure Data Lake Storage (ADLS) to dedicated SQL pools in Azure Synapse Analytics.
The following SQL Statement was run to create a target table in a dedicated SQL Pool. 
CREATE TABLE [dbo].[DimProduct]
(
[ProductKey] [int] NOT NULL,
[ProductLabel] [nvarchar](255) NULL,
[ProductName] [nvarchar](500) NULL
)
WITH
(
DISTRIBUTION = HASH([ProductKey]),
CLUSTERED COLUMNSTORE INDEX
 ); 
After that, a copy statement was run while connected to SQL dedicated pool and completed copying from ADLS.
But you find some data rows are not compressed as it was in the beginning. What will be the solution for this?

Answer options:

A.create single-columns statistics
B.Run “ALTER INDEX ALL ON [dbo].[DimProduct] REBUILD;”
C.Drop the target table and recreate
D.None of the above

Correct Answer: B
While loading data from Azure Data Lake Storage into dedicated SQL pools in Azure Synapse Analytics, there are two steps involved. One is creating the target table, and the next one is copying the data from ADLS to SQL pool. The CREATE TABLE command shows that the data is using columnstore indexing. So, it is possible for some data to be not compressed into a columnstore. To revolve this, we can simply rebuild the table to fore the columnstore index. This will increase the query performance as well.
Option A is incorrect: It is desired to optimize statistics by creating single-column statistics after loading the data into target table. But that will not solve the problem with columnstore compression in the target table.
Option B is correct: This command will rebuild the table and enforce the columnstore indexing. Thus the issue with compression will be removed.
Option C is incorrect: There is no need to drop the table and recreate, since the rebuild command will be the better choice.
Reference: To know more, please refer to the docs below: 

https://docs.microsoft.com/en-us/azure/synapse-analytics/sql-data-warehouse/sql-data-warehouse-load-from-azure-data-lake-store#optimize-columnstore-compression