A realization event occurs:

a. when the value of stock which an individual holds at year-end increases.

b. when a leasehold terminates and the property along with improvements made by the lessee revert back to the ownership of the lessor, as required by Helvering v. Bruun.

c. when a market transaction takes place with another party that alters the taxpayer's mix of assets and liabilities and results in a change in net worth.

d. All of the above.