Answer: These are the correct answers:
2. Hay MIL mapas en la biblioteca.
3. Hay DOSCIENTAS CINCUENTA Y NUEVE fotografías sobre el escritorio.
4. ¡Aquí hay DOS MILLONES DE dólares!
5. Sobre la mesa hay TREINTA Y UN cuadernos.
6. En la residencia hay CIEN estudiantes.
Explanation: Just to elaborate a little on the answer, it can be added that in this exercise they are asking you to fill the blank with the appropriate version of the number. The word "mapas" is masculine, so "mil una" should me discarded, and "miles" (thousands) cannot be used either, since, like in English, it requires the use of the preposition of ("de"). Likewise, "fotografías" is a feminine word, so you should discard "doscientos cincuenta y nueve" and "doscientos cincuenta y un." The words "miles" and "un millón" (one million) require the use of the preposition "de," so they cannot be used in option 4. "Treinta y uno" could be used after the question "How many "cuadernos" there are on the table," but if the number is followed by a noun the word "uno" should not be used. Finally, "cientos" (hundreds) and "miles" require the use of the preposition "de," so the correct option is "cien" (one hundred).