To determine how many houses 15 men can build in 45 days, given that it takes them 6.5 days to build one house, follow these steps:

1. Calculate the number of houses one group of 15 men can build in 45 days:

Since 15 men can build 1 house in 6.5 days, the number of houses they can build in 1 day is:

\[
\text{Houses per day} = \frac{1 \text{ house}}{6.5 \text{ days}} = \frac{1}{6.5}
\]

In 45 days, they can build:

\[
\text{Houses in 45 days} = 45 \times \frac{1}{6.5} = \frac{45}{6.5} = 6.923
\]

Rounding this to the nearest whole number, they can build approximately 7 houses in 45 days.

Therefore, the correct answer is:

B. 7 houses