Least penetrating power -

Ans. is 'c' i.e., Alpha particle Penetration power : Gamma rays> X rays > Beta particle > Alpha particle (or helium ion)Ionizing & damaging power : is reserve Alpha particle (or helium ion) > Beta particle > X ray> Gamma rayImportant facts about ionizing radiationso Alpha particles (Helium nuclei) have highest ionizing power because they have a large charge, o Alpha particles have the highest damaging power as they are relatively slow and heavy,o Gamma rays have highest penetrating power because they are high energy photons.o X-rays have 2nd highest (slightly less than g-rays) penetrating power,o Gamma rays have minimum ionizing power,o Gamma rays have least damaging power.o Order of penetrating power - Gamma rays > X-rays > b-particles > a-particles.o Order of ionizing & damaging power - a-particles > b-particles > X-rays > Gamma rays.