A - Use a <template> tag to display in-place error messages. A <template> tag is a way of rendering HTML content conditionally in Lightning web components. You can use a <template> tag with an if:true or if:false directive to show or hide an error message based on a property value. This way, you can display the error message within the component itself, and make it more visible and relevant to the user.

C - Use a Lightning web component to aggregate and display all errors. You can create a custom Lightning web component that acts as an error handler for all your other components. You can use events to communicate the errors from the child components to the parent component, and then use a <lightning-messages> component to display all the errors in a single place. This way, you can avoid showing multiple toast messages, and provide a consistent and centralized way of handling errors.

https://developer.salesforce.com/blogs/2020/08/error-handling-best-practices-for-lightning-web-components
https://developer.salesforce.com/docs/platform/lwc/guide/data-error.html
https://www.infallibletechie.com/2016/11/lightning-component-best-practices.html...