The correct answer is B. 35 mb.
The water vapor pressure is the partial pressure of water vapor in a mixture of gases. It is the pressure that water vapor would exert if it were the only gas present. The water vapor pressure can be calculated using the following equation:
$P_{w} = \frac{e_w}{P_{tot}}$
where $P_{w}$ is the water vapor pressure, $e_w$ is the water vapor pressure, and $P_{tot}$ is the total pressure.
In this case, the total pressure is 1,000 mb and the water vapor pressure is 3.5% of the total pressure, so the water vapor pressure is:
$P_{w} = \frac{0.035 \times 1,000}{100} = 35$ mb
Therefore, the water vapor pressure is 35 mb.
Option A is incorrect because it is the total pressure.
Option C is incorrect because it is the total pressure multiplied by 10.
Option D is incorrect because it is the total pressure minus the water vapor pressure.