A Large Dog Breed Traditionally Used as Guard Dogs

Puzzle 8 of Science Fair Projects Piece of Cake Pack

Question is: A Large Dog Breed Traditionally Used as Guard Dogs

and the answer is: MASTIFF

Word Meaning:

MASTIFF — One of an old breed of powerful, deep-chested, and smooth-coated dogs, used chiefly as watchdogs and guard dogs.

Definitions from Wiktionary.