Diagonal brace left in the basement by builder. Can I remove it? Replace it?
Yes, you can remove the brace because:


The wall is non-load bearing (not carrying a load)
The diagonal brace is not secured in a manner to transfer any load at the top or bottom
The diagonal brace is not secured to top plate
The diagonal brace is not secured significantly to vertical stud...the picture cuts off a portion of the brace that crosses the stud so I’d verify that
The diagonal brace is singular