The reason that oscillating systems are good to use for clocks is:
A. The period of an oscillation does not depend upon amplitude.
B. Real mass-spring systems are frictionless and energy is perfectly conserved.
C. The frequency of an oscillator does not depend on mass.
D. The frequency of an oscillator does not depend on the strength of the spring.
E. Oscillators are efficient at turning motion into heat.

A. The period of an oscillation does not depend upon amplitude.

Explanation:

The period of a spring-mass system is:

T = 1/f = 2π√(m/k)

where f is the frequency, m is the mass, and k is the spring constant.

The answer isn't B.  There are no frictionless systems in the real world.

The answer isn't C or D.  As shown, the frequency is a function of both the mass and the spring constant.

The answer isn't E.  Turning motion into heat is not an advantage for a clock.

The correct answer is A.  The period of the system does not depend on the amplitude.