Error spotting — Identify the erroneous segment (A/B/C) or mark D for no error: 'Last month we celebrated / the wedding of our sister for whom / we have been looking for a suitable alliance for three years. / No error.' Check tense sequencing with past time markers and eliminate tense inconsistency.

Given data

• A: Last month we celebrated
• B: the wedding of our sister for whom
• C: we have been looking for a suitable alliance for three years.
• D: No error.

Concept/Approach (sequence of tenses)
The adverbial 'Last month' fixes a finished past time. Any ongoing action that led up to that past point should be expressed with the past perfect (progressive): 'had been looking' (not present perfect progressive 'have been looking').

Step-by-step correction
C: 'we have been looking … for three years' → 'we had been looking … for three years'.Corrected sentence: 'Last month we celebrated the wedding of our sister for whom we had been looking for a suitable alliance for three years.'

Note
You may also streamline the relative clause to avoid repetitive 'for': '…our sister for whom we had been looking a suitable alliance …'; exam focus remains tense.

Final Answer
C — Replace with 'had been looking' to match 'Last month'.