Abcd code is being transmitted to a remote receiver. the bits are a3, a2, a1, and a0, with a3as the msb. the receiver circuitry includes a bcd error detector circuit that examines the received code to see if it is a legal bcd code (i. e., ≤1001). design this circuit to produce a high for any error condition.

BCD only goes from digit 0 (0000) to digit 9 (1001), because for 10 you need two digits, so all you've got to do is make a function that produces high for numbers from 10 (1010) to 15 (1111) as follows:
A3 A2 A1 A0   F
 0    0    0    0    0
 0    0    0    1    0
 0    0    1    0    0
 
 1    0    0    0    0
 1    0    1    0    1
 1    0    1    1    1
 1    1    0    0    1
 1    1    0    1    1
 1    1    1    0    1
 1    1    1    1    1
 
Then simplify the function: F = A3*A2 + A3*A1 Finally just draw or connect the circuit using NAND