The correct answer is: C. Obelisk : Fireman
A shale is a type of sedimentary rock, while a geologist is a scientist who studies the Earth’s physical structure and history. An obelisk is a tall, four-sided pillar with a square or rectangular cross section, while a fireman is a person who is trained to put out fires.
In both cases, the first word is a type of object or thing, while the second word is a person who studies or works with that object or thing.
A reef is a ridge of rock, coral, or sand that forms on the seafloor near or at the surface of the water. An astrologer is a person who studies the positions and movements of the stars and planets to predict the future.
Love is an intense feeling of deep affection. A philologist is a person who studies the history of languages and literature.
An aster is a type of flower. A botanist is a person who studies plants.
Therefore, the only pair of words that has the same relationship as the original pair of words is C. Obelisk : Fireman.