Final answer:
The central atom in the ball and stick model of a compound is typically the least electronegative element, which is often carbon, as it is less electronegative than other atoms like oxygen, and hydrogen is usually terminal.
Step-by-step explanation:
When using the ball and stick method of drawing a compound, the element that usually goes in the center of the model is commonly the one that is the least electronegative. Because carbon is less electronegative than oxygen, and hydrogen is normally terminal, carbon must be the central atom. Hydrogen and the halogens are almost always connected to only one other atom, indicating that they are usually terminal rather than central. Additionally, carbon atoms are often listed first in the chemical formula, such as in CCl4 and CO32-, which is another clue to the compound's structure. Therefore, the correct answer is (a) Carbon.