During which phase of meiosis do spindle fibers move the sister chromatids to the center of the cells?

The correct answer would be metaphase II.

Meiosis II is the second division of meiosis in which sister chromatids of chromosomes are separated.

It is also divided into four sub-stages:  

Metaphase II is the stage in which chromosomes are aligned at the central axis to form an imaginary line termed as the metaphase plate.

It is followed by anaphase II in which sister chromatids are separated and moved towards opposite poles.

Thus, the correct answer would be metaphase II.

the scientific method.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products