The isotopes k-37 and k-42 have the same
(1) decay mode
(2) bright-line spectrum
(3) mass number for their atoms
(4) total number of neutrons in their atoms

The isotopes K - 37 and K - 42 have the same bright line spectrum. 

Correct  The isotopes K-37 and K-42 have the same (2) bright-line spectrum

Reason:
K-37 and K-42 are isotopes of each other. Isotopes have same electronic distribution in their shell. The emission arises is due to deexcitation of electrons from excited state to ground state. Since, the electron distribution is same, both the isotopes i.e. K-37 and K-42, have same bright-line spectrum.