The correct answer is (B) $C1+$D1. Please see the explanation below:
Mixed cell referencing means when we create an excel formula such that on copy pasting the formula to another cell, either row number OR column name of the cell being referred to is not changed.
By adding a $ symbol before the column name or row number, you make it constant (or absolute) and it is not automatically adjusted if copied across cells. A mixed cell reference would have a $ symbol before either the column name or the row number, as is the case in option (B). So the formula in the option will always refer to column C and column D if you paste it across cells, only the row number reference will change
Further, in option (A) $C$1+$D$1, rows and columns of both C1 and D1 are absolutely referenced. In option (C) C1+D$1$, rows and columns of C1 are relatively referenced, and of D1 are absolutely reference. In option (D) C1+D1, rows and columns of both C1 and D1 are relatively referenced. Only in option (B) the columns are absolutely referenced and the rows are relatively referenced.