Oxygen-15 undergoes beta decay. Which particle correctly completes the equation to show that the numbers of nucleons on the two sides of the
equation are equal?
150 - 15N+ ?

Explanation:

Positron emission: In this process, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino. The beta positive particle released is basically a positron with +1 charge and no mass.

General representation of an element is given as:  

where,

Z represents Atomic number

A represents Mass number

X and Y represents the symbol of an element

For

it doesn't. it was imagined up in the middle ages.

the order for strong to weak forces such as

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

the fundamental forces are those forces that act between to objects or element to make interactions. there are four types of fundamental forces such as strong and weak nuclear forces (due to protons and electrons), electromagnetic force (due to the motion of electrons) and gravitational force (due to orbital movement of the planet). if these forces are arranged on the basis of their maximum strength then the order is

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force