Becl2(s) is correct.. because in solid state polymeric structure is present...

Answer:

Introduction:
In chemistry, the octet rule is a principle which states that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. In this question, we need to identify the molecule in which the central atom has a complete octet.

Solution:
The central atom in all given molecules is Be (beryllium). Let's examine each molecule one by one:

A. BeCl2(dimer):
In this molecule, the central Be atom has only four electrons around it, as it forms two single bonds with Cl atoms. Therefore, it does not have a complete octet.

B. BeH2(dimer):
In this molecule, the central Be atom has only four electrons around it, as it forms two single bonds with H atoms. Therefore, it does not have a complete octet.

C. BeH2(solid):
In the solid state, BeH2 forms a network structure in which each Be atom is bonded to four H atoms in a tetrahedral arrangement. Each Be atom has a total of eight electrons around it (two from each H atom and two from itself), fulfilling the octet rule.

D. BeCl2(solid):
In the solid state, BeCl2 also forms a network structure in which each Be atom is bonded to four Cl atoms in a tetrahedral arrangement. Each Be atom has a total of eight electrons around it (two from each Cl atom and two from itself), fulfilling the octet rule.

Conclusion:
Out of the given molecules, only BeH2(solid) and BeCl2(solid) have central atoms that fulfill the octet rule.