Please answer using 100 words or more.

The idea that employee stock options is an expense is something that is bedrock in U.S. GAAP. The matching of the expense is also basic theory that supports it. The valuation of "the option" is something that accountants are challenged by. Restricted stock units also come under the definition of compensation. The reason it is compensation is the fact it was earned during employment.

What are some methods to value the actual option itself?