Which one of the following phrasal verbs may be used in the blank space in the sentence?
We were happy in the beginning, but now all the troubles have _______ (Appeared).

Cropped up - something appearing or happening unexpectedly.
Come about - happen; take place.
Come out - to be in a certain condition or to achieve something at the end of a process or activity.
Rushed in - to move with great haste inward or into some place.
Out of these, given the context of the sentence, only 'Cropped up' is appropriate.
Thus, the complete sentence is:
We were happy in the beginning, but now all the troubles have cropped up.