***URGENT PLEASE HELP***

Roderick Usher's poem
By Edgar Allan Poe

I. In the greenest of our valleys,
By good angels tenanted,
Once a fair and stately palace—
Radiant palace—reared its head.
In the monarch Thought's dominion—
It stood there!
Never seraph spread a pinion
Over fabric half so fair.

II. Banners yellow, glorious, golden,
On its roof did float and flow;
(This—all this—was in the olden
Time long ago);
And every gentle air that dallied,
In that sweet day,
Along the ramparts plumed and pallid,
A winged odor went away.

III. Wanderers in that happy valley
Through two luminous windows saw
Figures moving musically
To a lute's well-tunèd law;
Round about a throne, where sitting
(Porphyrogene!)
In state his glory well befitting,
The ruler of the realm was seen.

Read this excerpt from Roderick Usher's poem:
Wanderers in that happy valley
Through two luminous windows saw
Figures moving musically
To a lute's well-tunèd law
What is the most likely explanation for what law means in this line?

a) Beat or structure
b) Limit or restriction
c) Vigor or enthusiasm
d) Volume or intensity