How to merge a specific commit in Git

You can use git cherry-pick to apply a single commit by itself to your current branch.

Example: git cherry-pick d42c389f

'git cherry-pick' should be your answer here.

Apply the change introduced by an existing commit.

Do not forget to read bdonlan's answer about the consequence of cherry-picking in this post:
"Pull all commits from a branch, push specified commits to another", where:

A-----B------C
 \
  \
   D

becomes:

A-----B------C
 \
  \
   D-----C'

The problem with this commit is that git considers commits to include all history before them

Where C' has a different SHA-1 ID.
Likewise, cherry picking a commit from one branch to another basically involves generating a patch, then applying it, thus losing history that way as well.

This changing of commit IDs breaks git's merging functionality among other things (though if used sparingly there are heuristics that will paper over this).
More importantly though, it ignores functional dependencies - if C actually used a function defined in B, you'll never know.