The highly efficient method of obtaining beryllium is :
A. Reduction of beryllium halide with Mg
B. Reduction of beryllium oxide with carbon
C. Dissociation of beryllium carbide
D. Electrolysis of fused beryllium chloride

Answer
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Hint: Be is metal which belongs to group 2 periodic table. Be is light and hard metal. It has the highest melting point and boiling point of all elements in group 2. The process of obtaining Be is very difficult.

Complete Step by Step Solution:
Beryllium is a rare metal but has several applications. It is a steel-gry, light and hard material. It is slowly oxidised in moist air. On heating, it burns with dazzling light. The process of obtaining Be is very difficult as it has a high affinity toward oxygen at high temperatures.

We have to find a suitable method for the extraction of Be.
A. reduction of beryllium halide with Mg
Beryllium halide has the common formula BeX2.
X is a halide like F, Cl, Br, etc.
This process leads to the formation of RMgX in high amounts. So, this is not an efficient method of obtaining beryllium.
So, A is incorrect.

B. reduction of beryllium oxide with carbon
Reduction of the oxide of metal using carbon at elevated temperature is smelting.
The oxides of lesser electropositive metals, for example, Pb, Zn, Fe, Sn, Cu etc. are reduced by this method. So, this is not an efficient method for obtaining beryllium.
So, B is incorrect.

C. dissociation of beryllium carbide
It disintegrates very gradually in water and methane gas.
\[{\rm{B}}{{\rm{e}}_{\rm{2}}}{\rm{C + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{2BeO + C}}{{\rm{H}}_{\rm{4}}}\]
It does not give Be on decomposition.
So, C is incorrect.

D. electrolysis of fused beryllium chloride
Beryllium chloride is an inorganic substance with the chemical formula BeCl2.
It has no colour and is a hygroscopic solid that undergoes dissolution in many polar solvents.
Its characteristics are identical to those of aluminium chloride, due to beryllium's diagonal relationship with aluminium.
Electrolysis of fused beryllium chloride gives beryllium.

The reaction happens as follows:
\[{\rm{BeC}}{{\rm{l}}_{\rm{2}}}\left( {{\rm{aq}}} \right) \to {\rm{Be}}\left( {\rm{s}} \right){\rm{ + C}}{{\rm{l}}_{\rm{2}}}\left( {\rm{g}} \right)\]
So, electrolysis of fused beryllium chloride gives beryllium.
So, option D is correct.

Note: Be is used in nuclear reactors. It is also used in making alloys. The presence of a small amount of Be in Mg reduced its combustibility. It has a high affinity for oxygen and forms beryllium oxide by combining it with oxygen. So, its most effective way of preparation is electrolysis of fused beryllium chloride gives beryllium.