What documents do I need to process to invite my son to come to the USA for a visit. He is 27 years old and married.

A letter to him inviting him is what you need. He will have to file his own application for a visa.

What to write in n 400 application

If he is in status when you file, you can indicate as such.

I485 denied as husband became a citizen

You applied for a derivative adjustment based on your husband's approved visa petition. Then he became a U.S. citizen. The decision was correct....

I had bought an apartment abroad before I immigrated to USA. Now if I would sell it do I need to pay income tax?

This is a question for an accountant.

Green card eligibility

In the U.S. you need an employer to file a visa petition for you, you cannot file on your own. See an attorney who can provide more information.

I live in CA and my case is in CA service center. I got a notice to get my biometrics done. I'm out of state for a few months

Most application support centers will do this for you.

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I -130 withdrawal

Not enough information.

Whether or not you can ask her to show her green card, you certainly do not have to employ her if you don't want to.

What is the best option for getting legal?

Not enough information. Your father should see an immigration lawyer to discuss his options.

Regarding visa transfer.

He is not authorized to work for both companies at the same time unless the petition is for concurrent employment.