When asking about real property tax relief credit, it ask about rent received being less then 20%. i did not receive any rental income. how do i answer the question?

Answer "Yes", since you received NO RENTAL INCOME and that would be LESS THAN 20%. 

It is one of the requirements of the credit. 

According to the State of New York:

"You are entitled to this refundable credit if you meet all of the following requirements for the tax year:

• you are subject to tax under Tax Law Article 22,
• you were a New York State resident for all of the tax year,
• your qualified gross income is $250,000 or less,
• you owned and primarily resided for six months or more of the tax year in real property that received the STAR exemption or that qualified you for the STAR credit,
• any rent you received from nonresidential use of your residence was 20% or less of the rental income, and
• you are the one that paid real property taxes on the property fully or partially."