George has always been cautious with his money. But, over the past two days, he has developed grandiose plans to bet his entire savings on a single horse race. With unrestrained exuberance, he has also been giving everybody he sees unsolicited advice on how to make millions in the stock market. George's behavior is most indicative of:

Mania.

Explanation:

Mania is a syndrome that results from an elevated mood, energy and behavior. It is a phase I of bipolar disorder. A person suffering from mania or manic syndrome is most likely to reveal the symptoms such as mood changes, sudden changes in activities and energy, changes in thought pattern, etc. These changes are most likely to occur because of emotional trauma, brain injury, or drug reaction.

In the given case, George's change in the behavior is an example of Mania or manic syndrome. George, who used to be very cautious with money, has suddenly shown changes in behavior, activities and energy. From saving money, he has become an extravagant, so much that he bet his entire savings on a single horse race.

So, the correct answer is mania.