Biology hormones and reproduction

my answer was B thickening of uterus wall as it preps for the menstruation but the answer key says menstruation directly

Hey, I think you are right. Increasing levels of progesterone cause thickening of uterus wall (endometrium), because it prepares uterus for zygote implantation. When fertilisation does not occur, corpus luteum starts turning into corpus albicans which no longer produces progesterone and thats the time when endometrium sheds, also called menstruation.