The correct answer is option 'D' - 18.

The long form of the periodic table, also known as the extended periodic table, consists of 18 groups. These groups are arranged in vertical columns, and they provide a way to categorize elements based on their chemical properties and electron configurations.

The 18 groups in the long form of the periodic table are further divided into four main blocks: s-block, p-block, d-block, and f-block.

1. s-block (Groups 1 and 2):
- Group 1 (alkali metals): This group includes elements such as hydrogen, lithium, sodium, and potassium. They are highly reactive metals and have one valence electron.
- Group 2 (alkaline earth metals): This group includes elements such as beryllium, magnesium, calcium, and strontium. They are also reactive metals but have two valence electrons.

2. p-block (Groups 13-18):
- Group 13 (boron group): This group includes elements such as boron, aluminum, gallium, and indium. They have three valence electrons.
- Group 14 (carbon group): This group includes elements such as carbon, silicon, germanium, and tin. They have four valence electrons.
- Group 15 (nitrogen group): This group includes elements such as nitrogen, phosphorus, arsenic, and antimony. They have five valence electrons.
- Group 16 (oxygen group): This group includes elements such as oxygen, sulfur, selenium, and tellurium. They have six valence electrons.
- Group 17 (halogens): This group includes elements such as fluorine, chlorine, bromine, and iodine. They have seven valence electrons and are highly reactive nonmetals.
- Group 18 (noble gases): This group includes elements such as helium, neon, argon, and xenon. They have eight valence electrons (except helium) and are nonreactive gases.

3. d-block (Transition metals):
- This block includes elements from Group 3 to Group 12, as well as the lanthanides and actinides. Transition metals have variable valence electrons and exhibit a wide range of chemical properties.

4. f-block (Lanthanides and Actinides):
- These elements are placed separately at the bottom of the periodic table. Lanthanides have atomic numbers from 57 to 71, while actinides have atomic numbers from 89 to 103. They are also known as rare earth elements and are mostly synthetic and radioactive.

In summary, the long form of the periodic table consists of 18 groups that categorize elements based on their properties and electron configurations. These groups are arranged in vertical columns and include the s-block, p-block, d-block, and f-block.