My son was born in April 2017 but he lived with us for only 3.5 months. Should i select "3.5" months or "Whole year" while reporting tax?

My son was born in April 2017 but he lived with us for only 3.5 months. Should i select "3.5" months or "Whole year" while reporting tax?

When i selected "4" months I was not getting Child Tax Credit.

I am not sure which one should i select