The basic physical unit of heredity

What is the meaning of the word checking as it is used in this sentence from the second paragraph of the passage?

Read the sentence. the great wave was inexpensive to purchase, costing only about the same as a double of noodles. since lots of copies could be printed and still look good, it made loads of money. which best revises this sentence to create a more formal style and tone? *the great wave was as cheap as buying a double of noodles. since lots of copies could be made and still look great, people started making piles of money. *the great wave was inexpensive to purchase, costing about the same as a double of noodles. since mass quantities could be produced and still look exquisite, it became highly profitable to sell. *the great wave cost next to nothing, practically the same as a double of noodles. since mass quantities could be produced and still look awesome, it became highly profitable to sell. *the great wave was inexpensive to purchase, similar to buying a double of noodles. since lots of copies could be made that still looked amazing, those who sold it made tons of money.

What is central purpose of the letter