The correct answer is option 3. The direction of the magnetic lines of force produced by a current-carrying wire can be determined using the right-hand thumb rule. According to this rule, if you point your right thumb in the direction of the current (up or down the wire), then the direction in which your fingers curl around the wire represents the direction of the magnetic lines of force.

Option 1, which states that the left-hand thumb rule is used for up-current and the right-hand thumb rule is used for down-current, is incorrect. The thumb rule is always based on the right hand, regardless of the direction of the current.

Option 2, which states that the right-hand thumb rule is used for up-current and the left-hand thumb rule is used for down-current, is also incorrect. The thumb rule is consistently based on the right hand for both up and down currents.

Option 4, which states that the left-hand thumb rule is used for both up and down currents, is not correct. The left-hand thumb rule is not commonly used in determining the direction of the magnetic lines of force for current-carrying wires.

Hence, the correct answer is option 3, which states that the right-hand thumb rule is used for both up