A. I looked under the bed - get this - and there it was!
Explanation:
One of the main purposes of using dashes is to add a dramatic tone and create a pause between the phrase in dashes and the rest of the sentence. Additionally, the parts that surround dashes have to be meaningful; dashes cannot interrupt a phrase. This happens in alternatives B, C and D. The only case where dashes can be used in this case is alternative A.
A. I looked under the bed - get this - and there it was!
Explanation:
The dash before and after "get this" is used to provide an emphasis, a direction to pay attention to "get this", and to separate it from the main sentence for further emphasis. It adds a level of emotion to the sentence, conveyed by the content within the dashes.
Furthermore, dashes are considered "emphatic" punctuations and hence their use here to draw attention is correct.
Other options are incorrect because they don't require dashes to convey their meaning or intent, and the content between them doesn't require added emphasis, in line with how they are structured.
