The stock of a target company is considered "in play" when:

A) an acquiring company begins working on a takeover plan.
B) an acquiring company makes a tender offer.
C) it becomes known that a company is an acquisition target.
D) a tender offer is endorsed by the target's management.
E) the acquiring company announces that it wants the target company.

The stock of a target company is considered "in play" when it becomes known that a company is an acquisition target. This means that the market has become aware that the target company may be acquired by another company, which can lead to an increase in the target company's stock price as investors anticipate a potential acquisition or bidding war. The other options (A, B, D, E) all involve specific actions taken by the acquiring or target company, but the stock is considered "in play" before these actions are taken, simply by virtue of the market speculation and attention surrounding the potential acquisition.