e. 1/2
With trig functions, multiple x values correspond with the same y value.
Using an initial x value (the principle value), we can find other x values for the same y value, this is what we are are being asked to find in this question.
There are slightly different ways to find these x values (also known as solutions) for each of the basic trig functions.
The x values are in degrees for the basic trig functions.
For cosine, the rule is as follows:
360 - principle value;
this will give what I, personally, like to think of as a secondary principle value (this value is not actually recognised as a secondary principle value, I just like to think of it as such). Anyway, all other solutions can the be found by adding or substrating any integer multiple of 360 to/from the PV and 'secondary PV'.
For your question:
cos60 = 1/2
60 is the x value (PV)
so...
360 - 60 = 300 is the 'secondary PV'
Just to be clear, this means that if I were to find the cos300, I would get 1/2.
That is sufficient for explaining the answer to this particular question but if you wanted to find any other solution, you would just have to do either:
60 + or - n(360)
or...
300 + or - n(360),
where n = any integer