Error: was / Correction: is
(The sentence states a general/universal fact, so the present tense 'is' is required, not the past tense 'was'.)
CBSE error-correction questions test tense usage. Since the sentence describes a permanent/scientific fact ("Latex is a natural polymer…"), the simple present tense must be used. The past tense 'was' incorrectly implies latex no longer exists or the fact is no longer true. Examiners expect the exact format: Error / Correction.