Question

In a mushroom, what is the visible umbrella-shaped portion known as

Answer 1

That is the reproduction part of the mushroom. It's the reproduction organ of this organism.

Answer 2

Aboveground macroscopic mushrooms organ is actually your reproductive organ, called a fruiting body.
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http://suite101.com/article/the-reproductive-life-cycle-of-a-mushroom-a114504

Answer 3

That is called Sporophore or fruit.

Answer 4

It's B in Plato