Earth is taken as zero electric potential, through it has net negative charge because of a. its layer radius
b. its smaller radius
c. heavy mass
d. its large permittivity

"Its larger radius"

Explanation:

The electric potential is given by :

Where

k is the electrostatic constant

Q is electric charge

r is the radius

For Earth, the value of r is very large and Q is very smaller. Due to large r, the potential of Earth's surface is almost zero. Hence, the correct option is (a).