Butter of tin is:
a.) \[SnC{{l}_{2}}.5{{H}_{2}}O\]
b.) \[SnC{{l}_{2}}\]
c.) \[SnC{{l}_{4}}\]
d.) \[SnC{{l}_{4}}.5{{H}_{2}}O\]

Hint: It was used as a chemical weapon in World War I, as it formed an irritating (but non-deadly) dense smoke on contact with air. It was substituted for by a mixture of silicon tetrachloride and titanium tetrachloride near the end of the war due to shortages of it.

Complete step-by-step answer:
We should know that tin tetrachloride or stannic chloride is a chemical compound with the formula $SnCl_4$. At room temperature it is a colourless liquid, which fumes on contact with air, giving a stinging odour. It is prepared from the reaction of chlorine gas with elemental tin.
It is prepared from the reaction of chlorine gas with tin at 115 °C.
\[Sn+2C{{l}_{2}}\text{ }\to \text{ }SnC{{l}_{4}}\]
We should note that Stannic chloride (\[SnC{{l}_{4}}\]) is hygroscopic and forms crystalline hydrates containing 3, 5, 6, and 8 molecules of water of crystallization. Such as:
\[SnC{{l}_{4}}.3{{H}_{2}}O,\text{ }SnC{{l}_{4}}.5{{H}_{2}}O,\text{ }SnC{{l}_{4}}.6{{H}_{2}}O,\text{ }SnC{{l}_{4}}.8{{H}_{2}}O\]
The pentahydrate \[SnC{{l}_{4}}.5H2O\], is known as "butter of tin". It is used as a mordant for dyes. We should note that a mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric. It may be used for dyeing fabrics or for intensifying stains in cell or tissue preparations. Although mordants are still used, especially by small batch dryers, it has been largely displaced in industry by direct.

Note: We should note that it was used as a chemical weapon in World War I, as it formed an irritating (but non-deadly) dense smoke on contact with air: it was substituted for by a mixture of silicon tetrachloride and titanium tetrachloride near the end of the War due to shortages of tin. It is also used in the glass container industry for making an external coating containing tin (IV) oxide which toughens the glass. Although a specialized application, \[SnC{{l}_{4}}\] is used in Friedel-Crafts reactions as a Lewis acid catalyst for alkylation and cyclisation. Stannic chloride is used in chemical reactions with fuming (90%) nitric acid for the selective nitration of activated aromatic rings in the presence of inactivated ones.