6. On a $3 \times 5$ chessboard, a piece can move one square at a time either horizontally or vertically, but not diagonally. Starting from some undetermined squares, the piece must pass through each small square exactly once, but it does not need to return to the starting square. Among these 15 small squares, ( ) can be the starting square for the piece.
(A) 6
(B) 8
(C) 9
(D) 10
6. B.

As shown in Figure 5, a $3 \times 5$ chessboard is colored in black and white.

In Figure 5, there are 8 black small squares and 7 white small squares. Since the chess piece alternates between black and white with each move, the 7 white squares cannot be the starting point.

On the other hand, any of the 8 black squares in Figure 6 can be the starting point.