Long day plants produces flowers when they exposed to :-
Any duration of light
Light period longer than a critical day length
Light period longer than 12 hrs
Light period shorter than a critical day length
The light period which is essentially required to induce flowering in plants is known as critical photoperiod. A long day plant will only flower when the light period is longer than a critical number of hours. Spinach, lettuce, and some varieties of wheat are long-day plants.
Hence the correct option is b.