Which pair below describes isotopes of the same element?

B) an atom with 6 protons and 6 neutrons - an atom with 6 protons and 7 neutrons

Explanation:An atom is the smallest particle of an element that can take part in a chemical reaction.Atoms consists of subatomic particles neutrons, electrons and protons.Atoms of the same element have an equal number of protons in their nucleus.Atoms may, however, differ in the number of neutrons. Such atoms that have a different number of neutrons but an equal number of protons are known as isotopes of an element.

The molecules contain like 54 of water 

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.