Correct Answer: NOMO

Solution : Given:
M_OMMN_M_NOMMN_M

To fill the series we have to divide the series – M_OM / MN_M / _NOM / MN_M
Let's check each option –
First option: ONMO; M O OM / MN N M / M NOM / MN O M (No repeated pattern has been found.)
Second option: NOMO; M N OM / MN O M / M NOM / MN O M (MNOM is repeated in the series.)
Third option: MONM; M M OM / MN O M / N NOM / MN M M (No repeated pattern has been found.)
Fourth option: NNMO; M N OM / MN N M / M NOM / MN O M (No repeated pattern has been found.)

So, the series becomes→MNOMMNOMMNOMMNOM. Hence, the second option is correct.