The smallest mass of a particle is called the rest mass, which is the mass of the particle when it is at rest. Therefore, option D is the correct answer to your question.
Rest mass, also known as invariant mass, is the mass of a particle when it is at rest relative to an observer. It is an intrinsic property of the particle and is independent of its velocity or momentum. The rest mass of a particle is usually denoted by the symbol "m" and is measured in units of energy divided by the speed of light squared (e.g., kilograms or electronvolts).