Final answer:
The smallest of the terrestrial planets with a cratered surface, a weak magnetic field, and tidally locked rotation is b. Mercury.
Step-by-step explanation:
The planet in question, which is described as the smallest of the terrestrial planets with a very old cratered surface, a weak magnetic field, and tidally locked rotation, is b. Mercury. Mercury's characteristics include being the smallest planet with a diameter of 4878 kilometers, less than that of Earth, and having a high density of 5.4 g/cm³.
Additionally, it is geologically inactive, similar to the Moon, and lacks significant geological activity due to its small size and being devoid of a substantial atmosphere. The geological history of Mercury, like that of the Moon, is preserved on its surface in the form of impact craters and other features due to the absence of geological renewing processes.