Correct Answer: NGIFSTR

Solution : Given:
ST_FIR_STNF_RGSTN_IRG__NFI_G

To fill the series, we have to divide the series – ST_FIR_ / STNF_RG / STN_IRG / _ _NFI_G

Let's check each option –
First option: NGSFITR; ST N FIR G / STNF S RG / STN F IRG / IT NFI R G (No pattern is repeated)
Second option: NGFISRT; ST N FIR G / STNF F RG / STN I IRG / SR NFI T G (No pattern is repeated)
Third option: NGIFSTR; ST N FIR G / STNF I RG / STN F IRG / ST NFI R G (STNFIRG is repeated in the series)
Fourth option: NGISFRT; ST N FIR G / STNF I RG / STN S IRG / FR NFI T G (No pattern is repeated)

So, only the third option follows a pattern. Hence, the third option is correct.