The correct answer is option 3: 18.

The modern periodic table is organized in periods, which are horizontal rows that indicate the number of electron shells in an atom. Each period corresponds to a different energy level.

The 5th period of the periodic table contains elements with five electron shells. Each shell can hold a specific number of electrons: the first shell can hold a maximum of 2, the second can hold a maximum of 8, the third can hold a maximum of 18, the fourth can hold a maximum of 32, and the fifth can hold a maximum of 50.

However, it`s important to note that the elements in the 5th period do not fill up all 50 possible spaces. In fact, they only occupy the first 18 spaces. Therefore, there are 18 elements in the 5th period of the modern periodic table.