The correct answer is B, $872. This can be determined by using the formula for simple interest: P(1+rt), where P is the principal, r is the annual rate of interest, and t is the number of years. In this case, we are given that P = 800 and r = 0.06 (6% expressed as a decimal). We also need to convert 18 months into years, so t= 1.5 years.

A savings account earns 6% simple interest per year. The principal is $800. What is the balance after 18 months?

Plugging these values into our formula gives us 800 * (1 + 0.06*1.5) = 800 * 1.09 = 872. Thus after 18 months, the balance in a savings account with an initial principal of $800 and 6% simple interest per year would be $872.