To solve this problem, we need to understand the mechanism of the contraceptive 'SAHELI'.$\newline$Let's analyze each option:$\newline \bullet$Option 1: Is an IUD.$\newline$ - 'SAHELI' is not an intrauterine device (IUD). It is an oral contraceptive pill.$\newline \bullet$Option 2: Increases the concentration of estrogen and prevents ovulation in females.$\newline$ - 'SAHELI' does not primarily work by increasing estrogen levels. It is a non-steroidal oral contraceptive.$\newline \bullet$Option 3: Blocks estrogen receptors in the uterus, preventing eggs from getting implanted.$\newline$ - 'SAHELI' contains Centchroman, which acts by blocking estrogen receptors in the uterus, thus preventing implantation of the fertilized egg.$\newline$ - This is the correct mechanism of action for 'SAHELI'.$\newline \bullet$Option 4: Is a post-coital contraceptive.$\newline$ - 'SAHELI' is not a post-coital contraceptive. It is taken regularly as a preventive measure.$\newline$Therefore, the correct option is Option 3: Blocks estrogen receptors in the uterus, preventing eggs from getting implanted.