Answer:
The type of error in the sentence is an explanation structure.
Step-by-step explanation:
The sentence presents an explanation structure error since it uses the words "The reason" and "because" which make it redundant, both expressions are used to give an explanation of the motives for the action, then to correct it, it is necessary to eliminate one of the options leaving either "The reason we are early is that my watch is fast." or "We are early because my watch is fast."