An interjection is a word or two (usually followed by an exclamation
mark), that expresses a sudden emotion. It is not grammatically
connected to the rest of the sentence.
Option A: 'Ouch' expresses a sudden pain, for example, if your friend pinches
your arm a little too hard, you immediately exclaim 'ouch!'.
Likewise, in the given sentence, the person has "slipped" as a result of which he experiences a sudden pain. So, option A is correct.
Option B: 'Alas' refers to sorrow or regret. It represents sad, unhappy emotions. But, it does not refer to physical pain as one would feel upon slipping.
C: 'Hurrah' represents joy or excitement. But, the author wouldn't express a positive emotion (joy) upon slipping.
Option D: 'Eh' expresses surprise or is used while questioning something.
These options are not appropriate for the given sentence. Hence B, C and D are incorrect.