Explanation:
This sentence expresses relief about something that happened in the past, so we need a past tense verb.

Let’s break it down:

“What a relief” – shows emotion about a completed action.

“...that the strike ended...” – this fits because “ended” is the simple past tense, and it matches the time frame of “when it did,” which also refers to the past.

Why the other options are incorrect:
A. would end – refers to a hypothetical or future event, not something that already happened.

B. was ending – past continuous, used for ongoing actions in the past; it doesn’t fit the sense of finality the sentence expresses.

D. has ended – present perfect, implies relevance to the present moment; it doesn’t fit with “when it did,” which refers to a specific time in the past.

E. is ending – present continuous, refers to something currently happening; doesn’t match the past context.