Final answer:
Historian Bede assigns symbols to the four evangelists: Matthew as a man, Mark as a lion, Luke as an ox or calf, and John as an eagle, but does not specify a 'sixth sign.' Option d.
Step-by-step explanation:
The sixth sign mentioned in the question is not explicitly identified in the provided historical context by Bede, but given the historical context provided, we can infer some connections.
According to the historian Bede, each of the four evangelists is represented by a specific symbol.
Matthew is represented by a man to suggest the human aspect of Christ, Mark by a lion symbolizing the triumphant and divine Christ of the Resurrection, Luke by an ox or calf representing Christ's sacrifice on the cross, and John by an eagle referring to Christ's second coming.
Therefore, the sixth sign could be interpreted as referring to one of these symbols. Since the ox or calf symbolizes Christ's sacrifice, as mentioned by Bede, option a. Lamb might be seen as closely related in symbolism to the sacrifice theme, but it is not one of the symbols used by Bede for the evangelists.
So option d is the correct answer.