Final answer:
Two elements bond with the same element in the same proportion because they are in the same column on the periodic table and share similar valence shell electron configurations, leading to similar chemical properties. So the correct option is D.
Step-by-step explanation:
The fact about elements that would help explain why two elements would bond with the same element in the same proportion is that the elements are found in the same column on the periodic table. This is because elements in the same column, or group, have similar valence shell electron configurations, leading to similar chemical properties. For instance, elements in Group 1A of the periodic table, also known as the alkali metals, such as lithium (Li) and sodium (Na), tend to form compounds in the same ratios as other elements and are very reactive due to having one valence electron. The organization of the periodic table, with elements listed by atomic number and grouped together based on similar chemical properties, helps to predict chemical reactions and to understand why certain elements bond similarly.