Why do we saya atomic spectra are like fingerprints of the elements?

Because no element can produce exactly the same result in an atomic spectra.
atomic spectra is the range of frequencies of light emitted by atoms .  { atomic spectra of light is unique spectra of light emitted by an element when electricity is run through it and  when it is viewed  through prism because they are unique they act as atoms fingerprint }   
atomic fingerprint is a term which is used to define characteristics of given element and can be used for identification 

Which of the following did you identify correctly

Hbr dissociates to form h+ and br-ca(oh)2 dissociates to form ca2+ and 2oh-the h+ and oh- react to form waterassuming the reactants are dissolved in water, the net reaction is h+ + oh- —> h2oif you’re asked to show all components of the reaction, the complete reaction is 2hbr + ca(oh)2 —> cabr2 + 2h2o