There are a number of acceptable answers.
For example, C, near-Earth TRANSPORT.
The Big Bang Theory Season 2, Episode 13
electron TRANSPORT in graphene.
The Big Bang Theory Season 7, Episode 6
...the TRANSPORT of electrons
through the nano-fabricated rings...
The Big Bang Theory Season 3, Episode 10
Let's see. Bradley tank.
Transport truck. Batmobile.
The Big Bang Theory Season 2, Episode 5