research, studies, surveys

The word "research" tends to be used in the singular, and as an uncountable word: 

• According to recent research, … 

• The research found the drug caused no apparent harm. 

Sources: 

• proofed.co.uk

• aje.com

You can use the words "studies" or "surveys" in the plural, if you feel you need to use a plural expression. 

Ways to refer to data and conclusions in studies, surveys and reports

Useful expressions for an essay

"Investigation" is not used in this context. Check its meaning here.