Methanoic acid & ethanoic acid have melting(deg. C) & boiling(deg. C) point of 8 & 101; & 17 & 118 respectively. An unknown...substance, X,...

Assuming it has to be either methanoic or ethanoic acid,
the most likely answer is ethanoic acid. Both the melting point and boiling point
experimental data are closest to the known values for ethanoic acid. The most likely
explanation for the slight differences is misreading the thermometer, a thermometer that
was not calibrated, or heating the substance too rapidly.  If no stirring was taking
place during the melting process and/or the boiling process there could have been
temperature variations within the bulk of the material that gave the indicated
readings.