Backup a single table I need to Truncate a table in Sql server which containing around 250GB data but before truncating i need to take backup of this,it may to csv file or other mode. What is best way to do it.
I'd recommend using the BCP utility.  From that documentation:

Copying table rows into a data file (with a trusted connection)

Basic

This example creates a data file named StockItemTransactions_character.bcp and copies the table data into it using character format.

At a command prompt, enter the following command:

bcp WideWorldImporters.Warehouse.StockItemTransactions out D:\BCP\StockItemTransactions_character.bcp -c -T