Explanation:

Correct Option: A
Following the common explanation, we can say that the code for ‘cool’ is ‘pa’.
Hence, the correct answer is option A.
Common Explanation:
weather is so cool → la pa ma se     …….(i)
so are we going → ma ne ta ra   …….(ii)
as going cool → pa ne he   …….(iii)
is weather hot → la se ka    …….(iv)
desert are hot → ka te ra  …….(v)
mountains are cool → pa ra ha   …….(vi)
From the equations (i) and (i), we get:
so → ma   …….(vii)
From the equations (i) and (iv), we get:
weather/is → la/se    …….(viii)
From the equations (i) and (iii), we get:
cool → pa  …….(ix)
From the equations (ii) and (iii), we get:
going → ne   ….(x)
From the equations (ii) and (v), we get:
are → ra    ….(xi)
From the equations (ii), (vii), (x) and (xi), we get:
we → ta    …….(xii)
From the equations (iii), (ix) and (x), we get:
as → he     …….(xiii)
From the equations (iv) and (v), we get:
hot → ka   ….(xiv)
From the equations (v), (xi) and (xiv), we get:
desert → te   ….(xv)
From the equations (vi), (ix) and (xi), we get:
mountains → ha   …….(xv)