Hi Sophia,
"Alternatives" are the suggested corrections. What's in the small box is what's in your document.
The suggestion is correct.
An apostrophe is used to indicate possession (Sophia's shoe, Barry's hat) or contraction (John's going out later, but he's home now—contracting "John is" and "he is").
1900s is the correct spelling to represent the ten year period from 1900 to 1909 or the hundred year period from 1900 to 1999.
Examples where "1900's" is correct would include this use as a possessive:
"Engine Number 1900's drivers had flat spots from an emergency stop on its last run."
Regards,
Barry