The field lines inside the solenoid are in the form of straight lines. This indicates that the magnetic field is the same at all points inside the solenoid. Thus, answer (iv) is correct

Introduction:
The magnetic field inside a long straight solenoid-carrying current is an important concept in electromagnetism. Understanding the behavior of the magnetic field in a solenoid is crucial to comprehend the working of various devices such as transformers and inductors. Let's explore the options given and determine the correct statement.

Options:
(1) The magnetic field inside a long straight solenoid-carrying current is zero.
(2) The magnetic field decreases as we move towards its end.
(3) The magnetic field increases as we move towards its end.
(4) The magnetic field is the same at all points.

Explanation:

1. Magnetic Field Inside a Long Straight Solenoid:
When a current flows through a long straight solenoid, it generates a magnetic field. This magnetic field is not zero but exists within the solenoid. The magnetic field lines are parallel to the axis of the solenoid.

2. Magnetic Field Behavior:
As we move towards the end of a long straight solenoid, the behavior of the magnetic field can be understood through the following points:

- Magnetic Field Concentration: The magnetic field lines inside a solenoid are closely packed and concentrated near the center of the solenoid. This concentration of magnetic field lines is due to the fact that the current-carrying wire in a solenoid is wound in the form of closely spaced turns.

- Magnetic Field Strength: The strength of the magnetic field inside a solenoid depends on various factors such as the number of turns, the current flowing through the solenoid, and the permeability of the material used for the solenoid core. However, the magnetic field strength inside a solenoid is relatively uniform along its entire length.

- Magnetic Field Outside the Solenoid: Outside the solenoid, the magnetic field is relatively weak and negligible. This is due to the fact that the magnetic field lines inside the solenoid tend to loop around within the solenoid itself.

Conclusion:
Based on the above explanation, we can conclude that the correct option is (4) - "The magnetic field is the same at all points." This is because the magnetic field inside a long straight solenoid carrying current is relatively uniform along its entire length, with the strongest concentration of magnetic field lines near the center of the solenoid.