The Munsell color notation with the lowest chroma from the options given is 10YR 1/2, which has a value of 1 and a chroma of 2, indicating a very dark and low saturation grayish color. The other notations provided have chromas ranging from 4 to 7, representing more colorful and saturated hues than 10YR 1/2. 7.5YR 3/1 has a chroma of 1 but was not one of the original options provided.