Error: vital (or was) / Correction: Water is vital to human health and fitness. (Present tense should be used for universal/general truths.)
The error is the use of past tense "was" for a universal truth. CBSE expects students to know that universal facts/general truths are always expressed in the simple present tense ("is"), not past tense ("was"). Clearly state the error word and the corrected sentence/word for full marks.