Which of the following is the most likely explanation for the circles in column D$?$: $*$

a$.$ The circled cells do not reference any other cells and therefore contain errors.

b$.$ The circled cells failed a data validation rule imposed on the data in column D$.$

c$.$ The circles were drawn in by the user and have no logical bearing on the data.

d$.$ The cells are circled as a result of conditional formatting that adds a circle icon to cells in column D with values below $25.$