Assertion (A):   $Co{F}_3$  is stable

Reason (R):  $Co{F}_3$  has high lattice enthalpy.

(A) is true but (R) is false

Both (A) and (R) are false 

Both (A) and (R) are true and (R) is the correct explanation of (A)

Both (A) and (R) are true and (R) is not the correct explanation of (A)