Answer:
none (as shown) or
one if all uppercase letters
Explanation:
A line of symmetry is a line you could place on a shape (usually a square, rectangle, triangle, etc, but here the "shape" is the word checkbook) which, if you folded the shape on the line, the two halves of the shape would exactly match each other.
"checkbook" does NOT have any lines of symmetry. BUT,
CHECKBOOK
does have a line of symmetry, horizontally, right thru the middle.