If we add $1000+0011=1011,$ what kind of addition is this?

a$.$ This is binary addition but not BCD addition, since the answer is an invalid BCD number.

b$.$ This is BCD addition but not binary addition, since the answer is an invalid binary number.

c$.$ This is an invalid operation for both binary and BCD$.$

d$.$ This is a valid operation for both binary and BCD$.$