Multiple choice questions on digital logic circuits, computer architecture, and binary arithmetic.

The first bit represents the sign (0 for positive, 1 for negative). The remaining bits represent the magnitude.
*   For 0110100:
    Sign bit is 0, so it's positive.
    Magnitude is $110100_2 = 1 \cdot 2^5 + 1 \cdot 2^4 + 0 \cdot 2^3 + 1 \cdot 2^2 + 0 \cdot 2^1 + 0 \cdot 2^0 = 32 + 16 + 4 = 52_{10}$.
    So, $0110100 = +52_{10}$.
*   For 01001100:
    Sign bit is 0, so it's positive.
    Magnitude is $1001100_2 = 1 \cdot 2^6 + 0 \cdot 2^5 + 0 \cdot 2^4 + 1 \cdot 2^3 + 1 \cdot 2^2 + 0 \cdot 2^1 + 0 \cdot 2^0 = 64 + 8 + 4 = 76_{10}$.
    So, $01001100 = +76_{10}$.

$$
+52_{10} - (+76_{10}) = 52 - 76 = -24_{10}
$$

The result is $-24_{10}$.
*   The sign bit for a negative number is 1.
*   The magnitude of 24 in binary:
    $24 \div 2 = 12$ remainder 0
    $12 \div 2 = 6$ remainder 0
    $6 \div 2 = 3$ remainder 0
    $3 \div 2 = 1$ remainder 1
    $1 \div 2 = 0$ remainder 1
    Reading remainders from bottom up: $11000_2$.
*   To match the 8-bit format of the options, the 7-bit magnitude for 24 is $0011000_2$.
*   Combining the sign bit and magnitude: $10011000_2$.

Comparing with the options:  
A. $00101000 = +40_{10}$  
B. $10101000 = -40_{10}$  
C. $00010100 = +20_{10}$  
D. $10011000 = -24_{10}$

The correct answer is **D**.