Which of the following is monoatomic?

The radius of the first orbit of hydrogen is $r_{H}$,and the energy in the ground state is $-13.6 \text{ eV}$. Considering a $\mu^{-}$-particle with a mass $207 m_e$ revolving around a proton as in a hydrogen atom,the energy and radius of the proton and $\mu^{-}$-combination respectively in the first orbit are (assume the nucleus to be stationary):

Two simple pendulums whose lengths are $1\,m$ and $121\,cm$ are suspended side by side. Their bobs are pulled together and then released. After how many minimum oscillations of the longer pendulum will the two be in phase again?

Which of the following statements is true?

The minimum value of $27 \tan ^2 \theta + 3 \cot ^2 \theta$ is

Which one of the following statements is $WRONG$ in the context of $X-$rays generated from an $X-$ray tube?