The correct option is B −2kp1p2cosθr3
We know that for a very small change in potential dV, potential energy can be written as,
U=−→p1⋅→E
But for a dipole with dipole moment p2 we can write that,
→E=2kp2cosθ^rr3+kp2sinθ^tr3
Since, dipole p1 is along the radial component of electric field
U=−p1×(2kp2cosθr3)cos0∘
⇒U=−2kp1p2cosθr3
Hence, option (b) is the correct answer.