To find the sum of -4.5 and 3.7, we can perform the addition:
\[ -4.5 + 3.7 \]
When adding a negative number and a positive number, we subtract the smaller absolute value from the larger absolute value and take the sign of the larger absolute value:
\[ 3.7 - 4.5 = -0.8 \]
Therefore, the sum of -4.5 and 3.7 is:
\[ -0.8 \]
So the correct response is -0.8.