The biological equivalent dose (BED) is defined as the product of the absorbed dose (in rads) and relative biological effectiveness (RBE).
Rem is the unit of biologically equivalent dose. So, optionis correct answer.
Rad is the unit of absorbed dose. So, option (a) is incorrect.
Gray is the unit of absorbed dose. So, option (b) is incorrect.
The unit of is equal to gray. So, the option (d) is incorrect.
Roentgens is the unit of exposure. So, option (e) is in correct.