It is a file function mode that is used to open a file for reading, writing or appending.
select one:
a. fopen()
b. feof()
c. fgets()
d. fgetc()

(a) fopen().

Explanation:

fopen() is used to open file different modes.It is used to open the file in read,wirte or append mode.

if the file is open in read mode first the file is opened successfully it is loaded in the memory and is ready to read.If the file is not opened successfully it return sNULL.

If the file is opened in write if the file exists it's content is overwritten else new file is created and content is added to it.

If the file is opened in append  mode the pointer is will reach at the last character in the file and then you can add text.

b. xor is the correct answer