Which is more acidic maleic or fumaric why??

Maleic acid is more acidic. pKa1 for maleic acid is 1.92 while for fumaric acid it is 3.03. The reason for the acidity of maleic acid is that the two -COOH groups interfere with each other in the unionized molecule because the double bonds force all the atoms (except the hydrogens in the OHs) into one plane. When one of the carboxyls loses a proton, the -COO- is free to rotate out of the plane of the other atoms so it no longer interferes with the other -COOH. Oddly enough Ka2 for fumaric acid is 4.44 as compared to 6.27 for maleic acid. You don`t get any further relief of steric hindrance after one of the carboxyl groups is ionized