Semiotics is the study of signs and symbols and their interpretation. In semiotics, a sign is anything that stands for something else. Signs are typically classified into three main types based on their relationship with the object they represent: icons, indices, and symbols.
Smoke is considered an indexical sign in semiotics because it has a direct, observable link to its cause, which is fire. The presence of smoke directly points to, or indicates, the presence of fire. This is a causal relationship, which is the defining characteristic of an index sign.
The other options represent different communication concepts:
Therefore, smoke functions as an indexical sign, signifying fire.
The faces of a cube are numbered 1 to 6. Four different positions of the cube are shown below :

The number on the face opposite to 2 is: