The correct answer is option 4 - parallel straight lines.

When electric current flows through a long solenoid, it produces a magnetic field. The magnetic field lines inside the solenoid are formed in the shape of parallel straight lines. This means that the field lines are aligned in the same direction and do not cross or diverge.

The reason for this is that a long solenoid is designed to have a uniform magnetic field inside it. The wire in the solenoid is wound in the form of a tightly packed coil, creating a cylindrical shape. When a current is passed through the wire, the magnetic field is generated around each loop of the coil, and these fields add up to create a uniform field along the axis of the solenoid.

The magnetic field lines being parallel straight lines indicate that the field strength and direction do not vary within the solenoid. This property makes solenoids useful in many applications, such as electromagnets, relays, and inductors.