Which sentence does not contain any errors in comma usage?

a.
the movie, that you are waiting for, is sold out.

b.
corn which can provide food for many, is grown in many kinds of soils.

c.
the picture that she is painting of the flowers will take months to complete.

d.
that item, which has been for sale for two weeks costs one hundred dollars.

The sentence that does not contain any errors in comma usage is option A. The movie, that you are waiting for, is sold out. The clause "that you are waiting for" is set off by a pair of commas because this is an additional information only and can be omitted in the sentence without affecting the thought of the whole sentence. This clause is called a subordinate clause. A subordinate clause cannot stand on its own and needs the main sentence. These are usually introduced by subordinating conjunctions or relative pronouns such as the word "that".