The number of elements in 5th and 6th period of periodic table are respectively
The number of elements in the 5th and 6th periods of the periodic table can be determined by understanding the pattern of electron filling in the atomic orbitals.

Explanation:
The periodic table is organized based on the electronic configuration of elements. Each period represents a new principal energy level or shell. The number of elements in a period corresponds to the number of atomic orbitals available in that energy level.

The 5th period of the periodic table corresponds to the n=5 energy level or the fifth shell. In this energy level, there are a total of 5 atomic orbitals - 1s, 2s, 2p, 3s, and 3p. Each orbital can hold a maximum of 2 electrons, resulting in a total of 2 + 2 + 6 = 10 electrons. Therefore, the 5th period of the periodic table has 10 elements.

The 6th period of the periodic table corresponds to the n=6 energy level or the sixth shell. In this energy level, there are a total of 6 atomic orbitals - 1s, 2s, 2p, 3s, 3p, and 4s. Similar to the 5th period, each orbital can hold a maximum of 2 electrons, resulting in a total of 2 + 2 + 6 + 10 = 20 electrons. However, it's important to note that the 6th period also includes the 4f and 5d sublevels, which can hold a maximum of 14 and 10 electrons, respectively. Therefore, the 6th period of the periodic table has a total of 10 + 14 + 10 = 34 elements.

Conclusion:
To summarize, the number of elements in the 5th and 6th periods of the periodic table are 10 and 34, respectively. Therefore, the correct answer is option C) 18, 32.