Correct Answer  Option 2 : the force of attraction between two unit masses separated by a unit distance
The correct answer is option 2) i.e. the force of attraction between two unit masses separated by a unit distance
CONCEPT:

Law of Universal Gravitation: It states that all objects attract each other with a force that is proportional to the masses of two objects and inversely proportional to the square of the distance that separates their centres.
It is given mathematically as follows:
\(F = \frac{Gm_1m_2}{R^2}\)
Where m1 and m2 are the mass of two objects, G is the gravitational constant and R is the distance between their centres.
 The gravitational constant G establishes a relationship between gravitational force, mass, and distance.
 The value of G is 6.67 × 1011 N kg2 m2.
EXPLANATION:
 From the law of universal gravitation, \(F = \frac{Gm_1m_2}{R^2}\)
 If we consider two unit masses separated by a distance of one unit i.e. R = 1 unit, m_{1} = m_{2} = 1 unit
\(⇒ F = \frac{G(1)(1)}{1^2}\)
⇒ F = G
 Thus, the universal gravitational constant numerically equals the force of attraction between two unit masses separated by a unit distance.