The correct answer is B. $A$1.
An absolute cell reference is a cell reference that does not change when you copy or move a formula. It is created by adding dollar signs ($) to the column and row references.
In option A, the exclamation point (!) is used to create a relative cell reference. A relative cell reference is a cell reference that changes when you copy or move a formula. It is created by omitting the dollar signs ($) from the column and row references.
In option C, the hash (#) is used to create a mixed cell reference. A mixed cell reference is a cell reference that is partially absolute and partially relative. It is created by adding a dollar sign ($) to either the column or row reference, but not both.
In option D, there are no dollar signs ($), so the cell reference is relative.
Therefore, the correct answer is B. $A$1.