Final answer:
The net electrostatic force on a test charge q placed between charges +Q and +2Q will be directed toward the larger charge, +2Q, due to the inverse square law of electrostatic forces.
Step-by-step explanation:
The direction of the electrostatic force on a test charge q placed between charges +Q and +2Q separated by distances d and 2d, respectively, can be determined by considering the force exerted by each charge separately. According to Coulomb's Law, a positive test charge will experience a repulsive force away from other positive charges. Since the charge +2Q is twice as large as +Q but twice as far away, the force from both charges on q will be the same magnitude but in opposite directions. However, due to the inverse square law, the force from +Q (being closer) will be stronger, thus the net electrostatic force on q will be directed towards the +2Q charge.