Learn English – “She is speaking” vs. “I am speaking”

Mutter is the only choice that implies the emotion we expect from the shopkeeper when he sees a customer he doesn't like.

The others would also work, but they don't imply the same negative emotion.

This belongs in the realm of style. If the people in examples are always male, then that tends to create a sense that we are excluding half the human race. When I need to use examples in any written communication or documentation, I use "she" in some examples and "he" in others. Always consistently, of course; the same example person does not shift between being a "he" and "she".

Traditionally, English used male pronouns in hypothetical examples about imaginary people. "Let he who is without sin cast the first stone." Using female examples is a modern way of eliminating sex bias from writing.

In writing from fifty or more years ago, you will not easily find these feminine examples. The male bias in the language used in the media decades ago is almost shocking.

For instance, take a look at page 8A in the advertizing section of the March 1940 Popular Mechanics. We find this:

In fact probably most of the men who study law today have no idea of taking the bar examination or becoming lawyers---they want law training to give the mastery of men and situations in business. You know that: (1) the man with legal training is a leader---not a follower [ ... ] (4) Many executive places are filled by men who have studied law. [...]

The words man and men appear in almost every sentence. The copywriter is devoid of any notion that women could study law or hold executive positions.