Correct Answer: BELMS

Solution : Given:
A_DC_FHGIJ_K_NPOQRT_

To fill the series we have to divide the series – A_ / DC / _F / HG / IJ / _K / _N / PO / QR / T_
Let's check the options –
First option: BEKMS; A B / DC / E F / HG / IJ / K K / M N / PO / QR / T S (No repeated pattern has been found.)
Second option: BELNU; A B / DC / E F / HG / IJ / L K / N N / PO / QR / T U (No repeated pattern has been found.)
Third option: BELMS; A B / DC / E F / HG / IJ / L K / M N / PO / QR / T S (The alphabets are ordered sequentially with every alternate part having the order of the alphabets reversed.)
Fourth option: BEKNU; A B / DC / E F / HG / IJ / K K / N N / PO / QR / T U (No repeated pattern has been found.)

So, the series becomes→ABDCEFHGIJLKMNPOQRTS. Hence, the third option is correct.