Final answer:
The gcc command with the '-o' option defines the output executable's name. For the given command 'gcc celsius.c -o Celsius', the compiled executable will be named 'Celsius'.
Step-by-step explanation:
When compiling a program using the gcc (GNU Compiler Collection) command-line tool, the "-o" option specifies the output file name for the compiled executable. In this case, the command gcc celsius.c -o Celsius will compile celsius.c and produce an executable file named Celsius. Therefore, the correct answer to what the executable file will be named is option c. Celsius.