Acid Catalysed Acetal Formation

Q. $C{H}_{3}CHO+H_{2}NOH-\to C{H}_{3}CH=N-OH$ The above reaction occurs at:

1. pH = 1
2. pH = 4.5
3. any value of pH
4. pH = 12

Q. Of the following, which compound is an acetal?

1. 
2. 
3. 
4.