The audience sat quietly as the maestro stretching his arms began to conduct the orchestra

answer: do

the 'italicized phrase' is a noun clause that is used as a direct object.

mark twain, real name samuel langhorne clemens, was an american writer, humorist, entrepreneur, publisher, and lecturer. among his novels are the adventures of tom sawyer and its sequel, the adventures of huckleberry finn, the latter often called "the great american novel". wikipedia