Final answer:
Centrioles migrate to form the poles of the mitotic spindle during mitosis, which is the correct context for this question, even though they also perform similar actions during meiosis I and II.
Step-by-step explanation:
The centrioles migrate to form the two poles of the mitotic spindle during mitosis and also during both stages of meiosis (meiosis I and meiosis II). During mitosis, the centrioles (part of the centrosome) start to move apart during prophase and are at opposite poles by the time the cell reaches prometaphase, which is when the spindle is fully formed. In meiosis, the process of spindle formation and centriole migration is similar.
In prophase I of meiosis I and in prophase II of meiosis II, centrioles also begin to migrate to opposite ends of the cell. However, when focusing on a single event of centrioles forming the two poles of the spindle, the correct answer in this context is (a) mitosis, as the role and process of the centrioles and mitotic spindle are most commonly associated with mitosis.