A frameshift mutation can occur when nucleotides are inserted or deleted in a number other than three, which disrupts the reading frame of the mRNA. In your options, "two nucleotides are deleted" or "two nucleotides are inserted" would both cause a frameshift mutation.
So, the correct answer is: two nucleotides are deleted or two nucleotides are inserted.
Since you have to choose one, "two nucleotides are deleted" is one specific way a frameshift can occur.