Final answer:
The statement true for all organic molecules is that they contain carbon. Organic molecules can be in various forms such as solids, liquids, or gases, and they might be water-soluble or insoluble. Water is not classified as an organic compound.
Step-by-step explanation:
The answer to the question 'Which statement is true regarding all organic molecules?' is A) They contain carbon. All organic molecules contain carbon, which forms the backbone to which hydrogen atoms or other functional groups are attached. The statement that organic molecules are always liquids (B) is incorrect as they can be solids, liquids, or gases. The notion that organic molecules are insoluble in water (C) is also false, as some organic compounds, like glucose, are water-soluble. Moreover, statement D, that organic molecules lack functional groups, is not true, as organic compounds often have one or more functional groups that affect their chemical properties. Lastly, water (B) is not one of the four major classes of organic compounds.