Select the correct option: The discovery, ___ was unexpected, altered the trajectory of the entire study.

Correct — B. The trigger is the pair of commas around the blank in 'The discovery, ___ was unexpected, altered'. They mark a non-defining relative clause, an aside about a noun that is already identified.

For a thing, a non-defining clause takes which. That cannot open a clause set off by commas.

'The discovery, which was unexpected, altered the trajectory of the entire study.' → option (b).

Why the others are wrong

• (a)
  that — That opens only defining relative clauses, which take no commas: the discovery that surprised us. The commas around this blank rule it out.
• (c)
  what — What means 'the thing that' and carries its own antecedent: what surprised us was the date. It cannot follow a noun that is already named, such as discovery.
• (d)
  whose — Whose is possessive and needs a noun after it: the discovery, whose results were unexpected. Here was follows the blank directly, so whose has nothing to own.

A relative clause describes a noun, and it comes in two kinds.

A defining clause tells you which one is meant: the discovery that changed the study. It takes no commas, and that or which both work for things.

A non-defining clause adds a fact about a noun the reader can already identify. It sits between commas, and for things it takes which, never that.

Test: delete the clause. If the sentence still points to the same discovery, the clause was non-defining.

Key facts

• Commas around a relative clause mark it as non-defining.
• For things, a non-defining clause takes which, never that.
• What means 'the thing that' and does not follow a named noun.
• Whose is possessive and must be followed by a noun: whose results.

Common traps

• Choosing that because it sounds natural, without checking for commas around the blank
• Choosing what, which cannot follow a noun already named such as discovery

No directly related past PYQ was found.