In watermelon, the most important indicator of maturity is

The correct answer is B: Dull sound when fruit is thumped

Here's why:

* Anthesis refers to the flowering stage. While knowing the days after flowering can be helpful, it's not the *most* reliable indicator because weather and growing conditions can affect maturity rates.

* The "thumping" method is a common way to check a watermelon's ripeness.
* A dull, hollow sound usually indicates the watermelon is ripe. The flesh has developed and is full of juice.
* A metallic or ringing sound often means the watermelon is unripe.

* Therefore, judging by the sound is the most practical and quick indicator for ripeness in watermelons.