Question

A set of atoms in an excited state decay
(A) in general, to any of the states with lower energy
(B) into a lower state only when excited by an electric field
(C) all together simultaneously in a lower state
(D) to emit photons only when they collide

Answer 1

Atoms become excited when electrons go from the ground state to an orbital level requiring more energy to exist there. This happens through some energy absorption whether that be heat, electricity or light.

Complete answer:
An excited-state atom is an atom in which the total energy of the electrons can be lowered by transferring one or more electrons to different orbitals. That is, in an excited-state atom not all electrons are in the lowest possible energy levels.
When an electron temporarily occupies an energy state greater than its ground state, it is in an excited state. An electron can become excited if it is given extra energy, such as if it absorbs a photon, or packet, of light, or collides with a nearby atom or particle.
When an electron in an atom has absorbed energy, it is said to be in an excited state. An excited atom is unstable and tends to rearrange itself to return to its lowest energy state. When this happens, the electrons lose some or all of the excess energy by emitting light.
A set of atoms in an excited state decays in general to any of the states with lower energy which is more stable than the excited state.
In conclusion, the right answer is option (A) in general, to any of the states with lower energy.

Note:
Excitation is an elevation in energy level above an arbitrary baseline energy state. This return to a lower energy level is often loosely described as decay and is the inverse of excitation. Long-lived excited states are often called metastable. The actual decay event of an individual atom happens randomly and is not the result of the atom getting old or changing through time.