The correct option is B (NH4)2BeF4
BeF2 is best prepared by decompostion of (NH4)2BeF4
(NH4)2BeF4⟶2NH4F+BeF2
Theory
Reactivity towards halogen:
Group 2 elements combine with halogens at elevated temperature to form their corresponding halides
M+X2⟶MX2
(X= F,Cl ,Br,I)
BeF2 is best formed by the thermal decomposition of (NH4)2BF4
(NH4)2BF4⟶2NH4F+BF2
BeO+C+Cl2⟶CO+BeCl2