The Rook - horizontal moves 1
If a rook starts a horizontal move on any rank it will be, by default, involved in a row of fields. The rooks move will end on the same rank unless it can take advantage of an empty field at the end of its move.
The rook on a16 can traverse the entire width of the board unimpeded and as all the fields are empty it could also choose to occupy any square on the fifthteenth rank.
The rook on a12 is blocked by a piece of its own colour on f12 and cannot pass so it can only end on squares b12-e12 and a11-e11.
The rook on a8 can also move across the width of the board passing by the white pawn on f7. As the rest of the fields are empty the rook could also occupy a7-e7 and g7-h7.
The rook on a4 is in the same position as the rook on a8. However, the black pawn on g3 cannot be taken although the rook could occupy g4 and share the field or pass by to h3/h4.