Final answer:
The D'mt kingdom, located in the Horn of Africa and existing between 700 and 400 BCE, traded with sub-Saharan African peoples as well as across the Red Sea with Arab merchants.
Step-by-step explanation:
Between 700 and 400 BCE, the kingdom that traded with sub-Saharan African peoples as well as across the Red Sea with Arab merchants was the D'mt kingdom. The D'mt kingdom was located in the Horn of Africa, roughly overlapping modern-day Eritrea, northern Ethiopia, and Djibouti.
It is known for its connections with the Sabean civilization of the southern Arabian Peninsula. This kingdom was predicated on agricultural practices, as well as trading connections that extended to both sub-Saharan Africa and across the Red Sea.
In contrast, other listed options such as Songhai, Kongo, and Zimbabwe were different African kingdoms from other periods, each notable in their own right for trade and cultural development within and beyond the African continent.