Carbon can form $p\pi-p\pi$ bond with which kind of elements?

Which species does not exist?

Explain the following reactions:
$(a)$ Silicon is heated with methyl chloride at high temperature in the presence of copper.
$(b)$ Silicon dioxide is treated with hydrogen fluoride.
$(c)$ $CO$ is heated with $ZnO$.
$(d)$ Hydrated alumina is treated with aqueous $NaOH$ solution.

Which of the following statements is not correct regarding $CO$?

The $C-C$ bond length is maximum in?

For the $14^{th}$ group (carbon family),which of the following statements is correct regarding the properties as the atomic number increases?