There is no keyboard shortcut for selecting a specific answer.
However, M Will auto-select the top post in the current viewport. Thus, you can often use it to flag any answer - scroll to the one you want, then press M and the F will open the flag menu for that answer. Example (spoilers to avoid the annoyance of constantly moving GIF):

But beware - it is the top post in the viewport. That might be one you barely see but is technically on top:

This can be a problem with bigger monitors, since you might never be able to auto-select the last answer, for example.
You can move the selection manually with
- J - next post (down). When nothing is initially selected, it select the top post in the viewport.
- K - previous post (up). When nothing is initially selected, it selects the bottom post in the viewport.
Both of these have the same issue that they work by viewport and thus they might not select what you want, depending on what is visible. And for larger monitors (looking at smaller posts), you might not even have a lot of control for what is visible. So, to pinpoint a specific post might be hard.
If building macros, you can have some control to select the Nth answer on the page. Start by U and then add J N times to get to that position in the answers. So, UJ for the first one UJJ for the second, etc.