The focus of an earthquake is:
a. on earths surface
b. the same as the epicenter
c. directly above the epicenter
d. along the fault

The focus of an earthquake is "along the fault".

Option D

Explanation:

The occurrence of earthquake is possible near the faults because of the presence of stress in huge amount. Therefore focus become the very first point of greatest movement only where all earthquake or seismic waves originate i.e "along a fault" and it lies below the surface.

While directly above the focus point or hypocenter of the surface, which face the most intense vibrations is understood as "epicenter". The earthquake occurs as the crust of the earth is moving continuously and have potential to create stress for changing the shape  of a body of rock.

the answer is b.) hydrogen bonds between bases.