Here i is directly related to the joint of p and r, and 2 is a constant . SO in this case , correct option is joint.
If it was i =2p or i = 2r, then it is direct and if it was i=2/p or i=2/r, then it is inverse.
But here it is i=2pr, that is i varies with the joint of p and r, so correct option is joint .