Usage of the word 'selected'

[Daemon99]

You might have seen the select boxes (combo boxes/dropdowns) on web sites.

You can select one of the options that it contains. I want to show a popup message if the user selects an invalid option.

My question is, which of the following messages can I use?

1. Please make sure the option selected is valid.

2. Please make sure the selected option is valid.

Can some one please help me with this?

On a separate note, which of the following sentences is correct?

1) I want to show a popup to the user which should contain a message.
2) I want to show a popup to the user that should contain a message.
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Non-native speaker of English
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I intend to live forever - so far, so good.

[Alan]

Hi Daemon,

On your first point I think it's really a question of balance or rhythm and in that light I would choose: