Barn is the unit of nuclear cross section. It is equal to:

A) 10^(-20) m^2
B) 10^(-28) m^2
C) 10^(-30) m^2
D) 10^(-14) m^2

The barn is a unit of area used in nuclear physics to describe the nuclear cross-section, which represents the effective area that a nucleus presents to an incoming particle (e.g., neutron, proton, or photon) during interactions.
Definition of a Barn:
1 barn (1 b1 \, \text{b}) is defined as:
1 b=10−28 m2.1 \, \text{b} = 10^{-28} \, \text{m}^2.
This unit was humorously named "barn" because it represents an area as "big as a barn" in the context of nuclear interactions.
Explanation of Options:
• Option A (10−20 m210^{-20} \, \text{m}^2): Incorrect. This value is too large for nuclear cross-sections.
• Option B (10−28 m210^{-28} \, \text{m}^2): Correct. This is the standard definition of a barn.
• Option C (10−30 m210^{-30} \, \text{m}^2): Incorrect. This is smaller than the standard barn unit.
• Option D (10−14 m210^{-14} \, \text{m}^2): Incorrect. This value is also far too large for nuclear cross-sections.
The barn is equal to:
10−28 m2.\boxed{10^{-28} \, \text{m}^2}.
Correct option: B.