Holding companies: A. allowed J. Pierpont Morgan to build a monopoly in the oil shipping business. B. are firms that control the stock of other companies. C. were firms where union membership was required. D. allowed J. Pierpont Morgan to build a monopoly in the oil shipping business.

B) are firms that control the stock of other companies.

Explanation:

A holding company is basically a company that owns other companies or stocks from other corporations. They basically do not produce any type of good or service by themselves.

Many times they are parent companies whose subsidiaries do produce and sell something, although they might also own only small percentages of other companies. E. g. Alphabet is a holding company whose main asset is Google but also owns Calico, CapitalG, Chronicle Security, DeepMind, DoubleClick, Google Fiber, GV, Jigsaw, Loon LLC, Makani Technologies, Sidewalk Labs, Verily, Waymo, Wing and X Development.