[n,d] = rat(YourNumericValue);
fprint(fid, '%d/%d', n, d)
However, if you have the numerator and denominator already available, then
fprintf(fid, '%d/%d', numerator, denominator)
which is what I think you probably want to do, if I remember your earlier question properly.
MATLAB is not designed to be able to tell the difference between 3/4 and 6/8: it will calculate the binary equivalents of the result and use that binary result, except in the Symbolic Toolbox where it will reduce the fraction to lowest terms and use the reduced rational fraction.