1. **Problem statement:** A block of mass $m$ and charge $q$ is connected to a fixed point $O$ by an inextensible string on a horizontal table. An electric field $E$ is applied perpendicular to the string and in the plane of the table. We need to find the tension in the string when it becomes parallel to the electric field. 2. **Setup and forces:** When the string is parallel to the electric field, the block experiences two main forces in the horizontal plane: - The tension $T$ in the string, directed along the string. - The electric force $F_e = qE$, acting on the charge in the direction of the electric field. 3. **Equilibrium condition:** Since the block is on a frictionless horizontal table and the string is inextensible, the block will be in equilibrium when the tension balances the electric force. The tension must provide the centripetal force to keep the block at rest in that position. 4. **Formula:** The tension $T$ in the string equals the electric force when the string is parallel to the electric field: $$T = qE$$ 5. **Explanation:** The electric field exerts a force $qE$ on the charged block. When the string aligns with the electric field, the tension must counteract this force to keep the block stationary. Hence, the tension equals the magnitude of the electric force. **Final answer:** $$\boxed{T = qE}$$