Final answer:
The 2018 social security wage base is C. $128,400, which is the maximum taxable earnings for Social Security tax for that year.
Step-by-step explanation:
The 2018 social security wage base is the maximum amount of earnings subject to the Social Security tax in a given year. The correct answer to the question 'What is the 2018 social security wage base?' is C. $128,400. This cap is adjusted annually based on inflation and changes in the average wage index, as per the Social Security Indexing Act of 1972.