The correct answer is option B, the right-hand thumb rule.

Explanation:
The right-hand thumb rule is used to find the direction of the magnetic field acting at a point near a current-carrying straight conductor. It states that if you point your right thumb in the direction of the current flow, then the curling direction of your fingers will give you the direction of the magnetic field lines around the conductor.

Here's a step-by-step explanation of how to use the right-hand thumb rule to determine the direction of the magnetic field:

1. Point your right thumb in the direction of the current flow in the conductor. This means that if the current is flowing from your hand towards your fingertips, your thumb should point in the same direction.

2. Curl your fingers around the conductor. The fingers should curl in the direction of the magnetic field lines around the conductor.

3. The direction in which your fingers curl represents the direction of the magnetic field lines. If you observe the conductor from the opposite side, the magnetic field lines will be clockwise. If you observe the conductor from the same side, the magnetic field lines will be counterclockwise.

The right-hand thumb rule is based on the principle of the magnetic field produced by a current-carrying conductor. When a current flows through a conductor, it creates a circular magnetic field around the conductor. The direction of this magnetic field can be determined using the right-hand thumb rule.

By using this rule, we can easily determine the direction of the magnetic field at any point near a current-carrying straight conductor. It is a simple and practical method that is widely used in physics and engineering to understand and analyze the behavior of magnetic fields in various devices and systems.

In conclusion, the right-hand thumb rule is a useful tool for determining the direction of the magnetic field near a current-carrying straight conductor. It helps in visualizing and understanding the behavior of magnetic fields in different situations, making it an important concept in the study of electromagnetism.