Note 4 – Stock-Based Compensation
Stock Incentive Program Descriptions
In January 2006, the Board of Directors adopted the ADTRAN, Inc. 2006 Employee Stock Incentive Plan (the “2006 Plan”), which authorized 13.0 million shares of common stock for issuance to certain employees and officers through incentive stock options and non-qualified stock options, stock appreciation rights, RSUs and restricted stock. The 2006 Plan was adopted by stockholder approval at our annual meeting of stockholders held in May 2006. Options granted under the 2006 Plan typically become exercisable beginning after one year of continued employment, normally pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date and had a ten-year contractual term. The 2006 Plan was replaced in May 2015 by the ADTRAN, Inc. 2015 Employee Stock Incentive Plan (the “2015 Plan”). Expiration dates of options outstanding as of December 31, 2019 under the 2006 Plan range from 2020 to 2024.
In January 2015, the Board of Directors adopted the 2015 Plan, which authorized 7.7 million shares of common stock for issuance to certain employees and officers through incentive stock options and non-qualified stock options, stock appreciation rights, PSUs, RSUs and restricted stock. The 2015 Plan was adopted by stockholder approval at our annual meeting of stockholders held in May 2015. PSUs, RSUs and restricted stock granted under the 2015 Plan reduce the shares authorized for issuance under the 2015 Plan by 2.5 shares of common stock for each share underlying the award. Options granted under the 2015 Plan typically become exercisable beginning after one year of continued employment, normally pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date and have a ten-year contractual term. Expiration dates of options outstanding as of December 31, 2019 under the 2015 Plan range from 2025 to 2026.
Our stockholders approved the 2010 Directors Stock Plan (the “2010 Directors Plan”) in May 2010, under which 0.5 million shares of common stock have been reserved for issuance. This plan replaced the 2005 Directors Stock Option Plan. Under the 2010 Directors Plan, the Company may issue stock options, restricted stock and RSUs to our non-employee directors. Stock awards issued under the 2010 Directors Plan become vested in full on the first anniversary of the grant date. Options issued under the 2010 Directors Plan had a ten-year contractual term. All remaining options under the 2010 Directors Plan expired in 2019.
The following table summarizes stock-based compensation expense related to stock options, PSUs, RSUs and restricted stock for the years ended December 31, 2019, 2018 and 2017, which was recognized as follows:
Table:
(In thousands) | 2019 | 2018 | 2017
Stock-based compensation expense included in cost of sales | $369 | $418 | $379
Selling, general and administrative expense | 3,889 | 3,989 | 4,063
Research and development expense | 2,704 | 2,748 | 2,991
Stock-based compensation expense included in operating expenses | 6,593 | 6,737 | 7,054
Total stock-based compensation expense | 6,962 | 7,155 | 7,433
Tax benefit for expense associated with non-qualified options, PSUs, RSUs and restricted stock | (1,659) | (1,432) | (1,699)
Total stock-based compensation expense, net of tax | $5,303 | $5,723 | $5,734


Question: Which year did all the remaining options under the 2010 Directors Plan expire?
Answer: 2019