Correct Answer: MNODROMD

Solution : Given:
R_ _ODRMN_ _ _MN_DR_NO_

To fill the series we have to divide the series – R_ _OD / RMN_ _ / _MN_D / R_NO_

Let's check each option –
First option: MNODROMD ; R MN OD / RMN OD / R MN O D / R M NO D (RMNOD is repeated in the series)
Second option: MNODRODM ; R MN OD / RMN OD / R MN O D / R D NO M (No repeated pattern has been found)
Third option: NMODROMD ; R NM OD / RMN OD / R MN O D / R M NO D (No repeated pattern has been found)
Fourth option: MNDOROMD ; R MN OD / RMN DO / R MN O D / R M NO D (No repeated pattern has been found)

So, the series becomes→RMNODRMNODRMNODRMNOD. Hence, the first option is correct.