Final answer:
The string length has the most significant effect on the period of a pendulum, as the period is proportional to the square root of the string length and independent of the mass of the bob and amplitude for small swings.
Step-by-step explanation:
The variable that caused the biggest change in the period of a pendulum is the string length. According to the formula for the period T of a simple pendulum, T is proportional to the square root of the length of the pendulum (L), and T = 2π√(L/g), where g is the acceleration due to gravity. This relationship shows that the period of a pendulum is directly affected by L, but it is independent of the mass of the bob and, for small amplitudes (less than about 15 degrees), the amplitude of the swing has negligible effect on the period.