Question

Which of the following sentences is punctuated correctly?

A.
The Leaning Tower of Pisa which started construction in 1173 did not begin sinking into the ground (until the third story was built).
B.
The Leaning Tower of Pisa—which started construction in 1173, did not begin sinking into the ground until the third story was built.
C.
The Leaning Tower of Pisa, which started construction in 1173, did not begin sinking into the ground until the third story was built.
D.
The Leaning Tower of Pisa which started construction in 1173 did not begin sinking—into the ground until the third story was built.

Answer 1

The answer is
C. The Leaning Tower of Pisa, which started construction in 1173, did not begin sinking into the ground until the third story was built

Answer 2

C - The Leaning Tower of Pisa, which started construction in 1173, did not begin sinking into the ground until the third story was built.

The use of commas sets off the dependent clause (which started construction in 1173).