Which of the following may be sensible expression? Given that A, B, C and D are all dimensionally different ?

Since A,B,C and D are all dimensionally different, dimensional formula of  AB  may be same as that of CD. If  AB and CD represent same physical quantity. They may be added and option–(1) is right. Again CD may be dimensionless. In that case eCD  is mathematically correct. Again A2B  may be dimensionless, in that case option –(2) is also correct. Since AB  and CD  can never be dimensionless, option–(3) is meaningless.