Let \( F \) be the number of French classes, then the number of Spanish classes will be \( 3F \) since there are three times as many Spanish classes as French classes.
The total number of classes can be represented as:
\[ F + 3F = 4F \]
Since there are a total of 850 students, we can divide the students based on the total number of classes:
\[ \text{Students per class} = \frac{850}{4F} \]
The number of students choosing Spanish will be:
\[ \text{Students in Spanish} = \text{Students per class} \times 3F = 3F \times \frac{850}{4F} = \frac{3 \times 850}{4} = \frac{2550}{4} = 637.5 \]
Rounding to the nearest whole number, we find:
\[ \text{Students in Spanish} \approx 638 \]
So the answer is: D. 638 students can take Spanish.