7/2
The expression that best estimates 6 3/4 divided by 1 2/3 is 7/2. 6 3/4 (or 6.75) divided by 1 2/3 (or 1.66...) comes out to 4 1/20 (or 4.05). The closest value to this in the options available is 7/2 (or 3.5). This answer is only .55 away from the correct value, while the rest of the options are far less close.