Select the correct option: Scarcely had the satellite deployed ___ it transmitted its first data packet.

Correct — B. The trigger is Scarcely had: a negative adverb at the front has pulled the auxiliary had in front of the subject.

Scarcely, like hardly, usually pairs with when to say the second event followed almost at once:
Scarcely had the satellite deployed when it transmitted its first data packet → option (b).

No sooner is the opener that pairs with than.

Why the others are wrong

• (a)
  than — Than belongs to the comparative no sooner: 'No sooner had it deployed than it transmitted'. Scarcely is not a comparative, so scarcely ... than mixes two patterns.
• (c)
  before — Before can also follow scarcely: Oxford Learner's Dictionaries gives 'I scarcely had time to ring the bell before the door opened'. It calls when the usual pairing, and when is what the key marks; see the dispute note.
• (d)
  until — Until marks how long something lasted, as in 'it stayed dormant until dawn'. It cannot close the scarcely pattern, which needs a word for the moment the second event happened.

Scarcely, hardly and barely are negative adverbs meaning 'only just'. With a past perfect they say one event happened almost immediately after another.

Put the adverb first and the clause inverts, as in a question: Scarcely had the satellite deployed, not 'Scarcely the satellite had deployed'.

The second clause is usually joined by when. No sooner does the same job but, being a comparative, takes than: No sooner had it deployed than it transmitted.

Key facts

• Scarcely, hardly and barely usually pair with when: Scarcely had I sat down when the phone rang.
• No sooner pairs with than: No sooner had I sat down than the phone rang.
• A negative adverb at the start of a clause brings inversion: Scarcely had it deployed.
• In this pattern the first clause takes the past perfect and the second the simple past.

Common traps

• Carrying than over from no sooner, because both patterns open with inversion and mean 'immediately after'
• Choosing before because it fits the meaning, when the key follows the usual pairing, scarcely ... when

No directly related past PYQ was found.