Final answer:
The atomic mass unit (amu) is defined in relation to the carbon-12 atom; thus, the value of an atom's mass in amu is close to its mass number. Given the provided options, the closest value to an atom's mass would be 14 amu, implying a hypothetical atom with a mass number close to 14.
Step-by-step explanation:
When trying to determine the mass of an atom that is closest to its true mass, it is essential to understand the concept of the atomic mass unit (amu). The amu is defined as one-twelfth of the mass of one carbon-12 atom, which is the isotope of carbon with 6 protons and 6 neutrons. Since each proton and neutron has a mass very close to 1 amu, and ignoring the much smaller mass of electrons, the mass of an atom in amu is generally close to its mass number (the total number of protons and neutrons).
Upon consideration of the above explanation, carbon-12 has a mass exactly equal to 12 amu by definition. This means that the value closest to the mass of the atom, considering a typical element with a similar number of protons and neutrons, would be an integral multiple of amu close to the mass number. Thus, the closest value to the mass of an atom from the options given would be 14 amu, which suggests an atomic composition of 7 protons and 7 neutrons, or a closely related mass number.