It would be kind to accept cell phone or mobile phone but it's not completely accurate. In the case of Crowe it was a landline and in the case of Campbell it was a cell phone. Both are "telephones".
I think the question refers to which character made their first appearance in "A Study in Scarlet," not which character was first mentioned in the book, although it could be interpreted as that. Maybe the question could be rephrased.
It definitely means "what character makes their first appearance," but I do think that Dr. Watson should be an acceptable type-in. He may not be as famous as Holmes, but he's still a very well-known fictional character. Or maybe specify "fictional detective" rather than "character"?
I tried Watson before Holmes just to see if it would work, and because I couldn't remember any of the other characters, even the policeman who comes back in other stories. No such luck though.
There is not "the" Tesla Gigafactory. They also produce cars in some factories - which are called gigafactory as well. In fact 2 of the 5 are for the production of cars (Del Valle & Grünheide, Germany). One of the two gigafactories responsible for production of batteries also produces vehicles.
The Tesla Gigafactories are as follows
Gigafactory 1: Batteries in Nevada
Gigafactory 2: Photovoltaic in Buffalo
Gigafactory 3: Batteries and Cars in Shanghai
Gigafactory 4: Cars in Brandenburg
Gigafactory 5: Cars in Texas
I'd love to have "Cars" as a possible solution...