In the poem "The Star-Spangled Banner" the use of the word fitfully is in the lines:
On the shore dimly seen through the mists of the deep,
Where the foe's haughty host in dread silence reposes,
What is that which the breeze, o'er the towering steep,
As it fitfully blows, now conceals, now discloses?
On the shore the enemy is difficult to see due to the mist. They become visible when the mist is lifted for brief periods of time in an irregular manner by the breeze.
The definition of the word fitfully which is used as an adjective is "coming, appearing, acting, etc., in fits or by spells; recurring irregularly."
Fitfully means irregularly. None of the other options scorning, sparked, greeted or terrifying have the same meaning as the word.