I do not believe a comma is necessary in this sentence at all. In fact, if you were to insert a comma after the word grazed (as you maybe tempted to do) it could change the meaning of the sentence. If you meant the lions were oblivious of the presence of the gazelles, you would insert a comma after grazed. But if you meant the gazelles were oblivious of the presence of the lions, you would leave the comma out.
The answer would be c - interjection.
Interjections are words or phrases that express emotion.
Examples would be wow, awesome, ummm, etc.
These are not appropriate for formal writing but used constantly in daily communication.
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