Final answer:
Among the options provided, the malignant, fast-growing tumor is an immature teratoma, known for its embryonal-type cells that can rapidly divide and spread.
Step-by-step explanation:
To determine which teratoma is a malignant, fast-growing tumor, we need to understand the nature of each teratoma listed. A mature teratoma typically consists of well-differentiated, benign tissues. A monodermal teratoma or struma ovarii partakes traits of being specialized to one type of tissue; in the case of struma ovarii, it usually focuses on thyroid tissue, often benign.
An ovarian carcinoid is also typically benign, mostly slow-growing and made up of neuroendocrine cells. However, an immature teratoma is characterized by the presence of immature, embryonal-type cells, which has the potential to be malignant and can grow and spread quickly. Consequently, the final answer in two line explanation in 300 words is that the malignant, fast-growing tumor among the options provided is an immature teratoma.