Gamma radiations are a type of electromagnetic radiation that are emitted from the nucleus of an atom. They are high-energy photons with a very short wavelength, and they have several important properties that distinguish them from other types of radiation.

a) Gamma rays are high-energy photons with a very short wavelength:
Gamma rays are the most energetic form of electromagnetic radiation. They have wavelengths that are shorter than those of X-rays, and they are produced by different processes than X-rays. Gamma rays are high-energy photons that are emitted from the nucleus of an atom during radioactive decay or nuclear reactions. They have no mass or charge, and they travel at the speed of light.

b) Gamma emission changes the atomic number:
This statement is not true. Gamma emission does not change the atomic number of an atom. The atomic number of an atom is determined by the number of protons in its nucleus, and gamma emission does not involve the addition or removal of protons. Instead, gamma emission is a process in which an excited nucleus releases excess energy in the form of a gamma ray. This process does not alter the composition of the nucleus or change its atomic number.

c) Gamma emission does not change the atomic mass:
This statement is true. Gamma emission does not change the atomic mass of an atom. The atomic mass of an atom is determined by the combined mass of its protons and neutrons, and gamma emission does not involve the addition or removal of protons or neutrons. Instead, gamma emission is a purely energetic process that releases excess energy from an excited nucleus.

d) They have high penetrating power as compared to alpha and beta radiations:
This statement is true. Gamma rays have much higher penetrating power than alpha and beta particles. Alpha particles are heavy and positively charged, so they interact strongly with matter and have a limited range. Beta particles are smaller and negatively charged, so they have greater range and penetration than alpha particles but still interact with matter to a significant extent. In contrast, gamma rays have no mass or charge, so they can pass through matter with little interaction. They can travel long distances through air, penetrate solid materials, and require thick shielding to be stopped.

In conclusion, the statement that is not true about gamma radiations is option 'b': Gamma emission does not change the atomic number. Gamma emission is a process that releases excess energy in the form of high-energy photons without changing the atomic number or atomic mass of an atom.