Question

The nominal gdp of the u.s. in 2015 was approximately​ $17.3 trillion. this means that

Answer 1

This means that:

-the value of output in 2015 was around $17.3 trillion
-total income in 2015 was around $17.3 trillion
-total spending in 2015 was around $17.3 trillion

Value of output and total spending has basically similar meaning because they account for the total amount required to produce all output/product within the period.
Assuming that it's all gonna be sold, the total income will be at least close to the amount needed for production because the nominal GDP is evaluated at current market prices.