4) is the correct option.
You can take a example and check easily
let array a contains elements : 3,5,7
and array b contains elements: 2,6,8,9
after merging array c will contain elements as 2,3,5,6,7,8,9
cheking option 1) a[i] ≥ b [i] => c[2i] ≥ a [i]
for elements at zero index in both the arrays, the one which is smaller is placed in array c at zero index.. so this fails as c[0]=2 and a[0]=3
2)a[i] ≥ b [i] => c[2i] ≥ b [i]
taking i=1
a[1]=5 , b[1]=6 and c[2]=5 so this condition also fails as c[2]<c[1](this condition may sometimes be true but not always, in case if array a and array b contains elements in common)
3)a[i] ≥ b [i] => c[2i] ≤ a [i]
a[1]=5 , b[1]=6 , c[2]=5 and so this condition is true as c[2]=a[1]
4)a[i] ≥ b [i] => c[2i] ≤ b [i]
a[1]=5 , b[1]=6 and c[2]=5 , viz also true as c[2]<b[1]