Is C2H2 planar molecule?Are linear moecules planar? Why?Thanks very much🙏?

Yes, ethylene is planar. That means that all four of the atom are in the same plane. It is 'forced' into this orientation by the triple bond between the carbons which keeps the two hydrogen atoms at either end from aligning in any other direction,

Linear molecules (formed between a pair of atoms like carbon monoxide) are always planar. Simplistically, you can always form a plane that passes though two points (in this case, atoms).

This is acetylene and it is LINEAR, not planar. Diatomic molecules are the same; they are not planar -- an INFINITE number of planes pass through a line.