The correct statement is:
B. Molecules are added and removed during chemical reactions.
Here's a brief explanation of why the other statements are not true:
A. Atoms always remain intact during a chemical reaction - This is misleading; while atoms themselves are not created or destroyed in chemical reactions (according to the law of conservation of mass), they might be rearranged to form new molecules.
C. Molecules always remain intact during chemical reactions - This is false because, during a chemical reaction, bonds between atoms in molecules can break and new ones can form, resulting in the creation of new molecules.
D. Atoms are added and removed during chemical reactions - This is also false; atoms are rearranged rather than added or removed. The total number of atoms remains constant throughout the reaction.
Therefore, option B accurately reflects the behavior of molecules in chemical reactions.