investment A has 500 (for April) + 600 (for May) + 400 (for June) with a total of 1500.
investment B has 400 (for April) + 300 (for May) + 500 (for June) with a total of 1200
Converting each amount to ratios gives investment A: 33%, 40%, and 27% respectively. Whereas investment B ratio is: 33%, 25%, and 42%. Hence, the largest month to month change is June.