The correct answer is option 'B' - Newton-second.

**Explanation:**

**Momentum:**
Momentum is a fundamental concept in physics that describes the motion of an object. It is defined as the product of an object's mass and its velocity. Mathematically, momentum (p) is given by the equation:

p = m * v

where p is momentum, m is mass, and v is velocity.

**SI Unit of Momentum:**
The momentum has a derived SI unit, which is obtained by multiplying the units of mass and velocity. In the SI system, the unit of mass is kilogram (kg) and the unit of velocity is meter per second (m/s). Therefore, the unit of momentum is:

kg * m/s

This unit is called the kilogram-meter per second (kg·m/s). However, it is also represented by the unit Newton-second (N·s).

**Explanation of the Unit:**
To understand why the unit of momentum is represented by Newton-second, we need to consider Newton's second law of motion. According to this law, the force acting on an object is equal to the rate of change of its momentum. Mathematically, it can be written as:

F = dp/dt

where F is force, dp is the change in momentum, and dt is the change in time.

From the equation, we can see that the change in momentum (dp) is equal to the force (F) multiplied by the change in time (dt). The SI unit of force is Newton (N), which is defined as kg·m/s^2. Therefore, the unit of momentum can also be expressed as:

N * s

This unit is equivalent to the kilogram-meter per second (kg·m/s) mentioned earlier.

Hence, the SI unit of momentum is Newton-second (N·s), as it represents the product of force (N) and time (s) required to change the momentum of an object.

In conclusion, the correct answer is option 'B' - Newton-second, as it accurately represents the SI unit of momentum.