The answer is option C. Number of double covalent bonds between carbon atoms is responsible for causing different shapes of saturated and unsaturated fatty acids. The double covalent bond causes kink in the unsaturated fatty acids because of which their stacking is disrupted and they become loosely packed. The problem with double covalent bond is that unlike single covalent bond it can't rotate and has almost cis-configuration. This is also the reason behind their liquidity at room temperature.