Final answer:
The structure indicated by the black dashed arrow in the diagram is a centriole, a key part of the centrosome involved in organizing microtubules during mitosis.
Step-by-step explanation:
The structure in red, indicated by the black dashed arrow on the diagram of a centrosome and associated microtubules, is a centriole. A centriole is a cylindrical cell structure composed of nine triplets of microtubules, and two centrioles are found at right angles to each other within the centrosome. During mitosis, the centrosomes serve as microtubule-organizing centers for the mitotic spindle. Therefore, to answer the question, the identity of the structure in red is option A. centriole.