The correct choice is (d) Elemental chlorine free
The explanation: The term ECF in chlorine bleaching stands for elemental chlorine free bleaching. Chlorine bleaching was identified as the major source of polychlorinated dioxins and dibenzofurans compounds. In addition to dioxins and furans, a host of other chlorinated organic compounds (known collectively as adsorbable organic halides or AOX) are formed during chlorine bleaching. Strict regulations now limit the production of these chlorinated compounds, resulting in a move away from molecular chlorine bleaching to chlorine dioxide, so-called ECF or elemental chlorine free bleaching.