The correct answer is:
8 ways.
Explanation:
She will have at least one row with 2 plants, so using 17 rows with 1 plant each will not work.
She can have 15 rows with 1 plant each and 1 row with 2 plants.
She can have 13 rows with 1 plant each and 2 rows with 2 plants.
She can have 11 rows with 1 plant each and 3 rows with 2 plants.
She can have 9 rows with 1 plant each and 4 rows with 2 plants.
She can have 7 rows with 1 plant each and 5 rows with 2 plants.
She can have 5 rows with 1 plant each and 6 rows with 2 plants.
She can have 3 rows with 1 plant each and 7 rows with 2 plants.
She can have 1 row with 1 plant and 8 rows with 2 plants.
This makes 8 different ways to plant these tomatoes.