Explanation:

When zinc is added to dilute sulfuric acid, it reacts to produce hydrogen gas. This reaction is vigorous and produces a lot of fizzing. When zinc is added to concentrated sulfuric acid, the reaction is much slower and produces less gas. This is because the concentrated acid is more viscous and does not allow the zinc to dissolve as easily. When zinc is added to acetic acid, the reaction is very slow and produces very little gas. This is because acetic acid is a weak acid and does not react with zinc as easily as sulfuric acid.

Therefore, the fizzing will be the least when zinc is added to concentrated sulfuric acid.