Answer is: C. Krypton is a noble gas, so it was difficult for chemists to find it, as though it was a secret.
Krypton is a chemical element with symbol Kr and atomic number 36. 
Krypton has 36 electron and 36. It is noble gas (group 18). 
Electron configuration of krypton atom: 
₃₆Kr 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶. 
Noble gases are in group 18: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). They have very low chemical reactivity.