21. When a cell releases a signal molecule into the environment and a number of cells in the immediate vicinity respond, this type of signaling is
(A) typical of hormones.
(B) autocrine signaling.
(C) paracrine signaling.
(D) endocrine signaling.
(E) synaptic signaling.

The correct answer is **(C) paracrine signaling**.

### Explanation:
**Paracrine signaling** is a type of cell signaling where a cell releases a signal molecule that acts on nearby cells in the immediate vicinity. The signal does not travel long distances and only affects cells that are close to the signal-releasing cell.

Here’s an overview of the other types of signaling:

- **(A) Hormonal signaling**: This involves signal molecules (hormones) being released into the bloodstream and traveling long distances to target cells that have specific receptors for those hormones. This is an example of **endocrine signaling**.

- **(B) Autocrine signaling**: In autocrine signaling, a cell releases a signal molecule that binds to receptors on the same cell, influencing its own behavior. This typically involves the cell signaling to itself.

- **(D) Endocrine signaling**: In this form of signaling, hormones are secreted by endocrine glands and travel through the bloodstream to act on distant target cells in the body.

- **(E) Synaptic signaling**: This involves signaling between nerve cells (neurons) at synapses, where neurotransmitters are released to transmit signals between cells across a synapse.

Thus, when a cell releases a signal molecule into the environment, and the signal affects nearby cells, it is called **paracrine signaling**.