Select the most appropriate meaning of the given idiom.

Come rain or shine

under any circumstances

The most appropriate meaning of the idiom "come rain or shine" is under any circumstances.

The idiom is used to describe someone who is willing to do something or be somewhere, regardless of the weather or other conditions. For example, you might say that someone is going to a friend's party come rain or shine. 

The other options are not as accurate descriptions of the idiom.

• In favourable conditions suggests that the person is only willing to do something or be somewhere if the conditions are good, which is not the case with the idiom "come rain or shine".
• In difficult situations suggests that the person is only willing to do something or be somewhere if the situation is difficult, which is not always the case with the idiom "come rain or shine".
• During bad weather suggests that the person is only willing to do something or be somewhere if the weather is bad, which is not always the case with the idiom "come rain or shine".