(b) That's right. The correct answer depends on what meaning is given to the word 'during'.
When courts decide what words in a contract mean, the starting point is that words should be given their ordinary and natural meaning in the context of the contract as a whole. The ordinary and natural meaning of words can be found in a good dictionary.
A dictionary defines the meaning of 'during' as 'throughout' or 'for the period of'. Used without qualification, this word does not suggest that periods of notice are excluded from the period of the lease. The obligation to make alterations would therefore continue during the period of notice.