Answer:
We cannot begin to know how this affects your outlook for the future.
Step-by-step explanation:
In order to set a serious tone, use of colloquial language must be avoided in all cases - unlike the first option, which contains expressions such as "whoa!".
Additionally, whenever the serious tone is set, it will be perceived as such by every participant in the conversation; it doesn't need to be explicitly announced, such as in the second option with the word "seriously".
Declaring an environment where no extreme emotions avail (and done so with certainty) is the correct way to set a serious tone, as done in the third option.