Which Science Elective is easier class to take between biology, chemistry, environmental science, integrated science and doesn't required a lab?
The answer to the question of which class is easier will depend on the school and level. You can look at your school’s course catalog and see how many units the class is worth, and if a lab is required. Talk to your advisor about this. Most schools include general education introductory classes that help non-science majors meet the science requirement without difficult coursework.
Of your options, integrated science is more likely to be a lower level class. This is often a survey class, involving a sampling of different scientific fields. Biology, chemistry, and physics all require labs and are content-heavy. Environmental science is also popular with non-science majors, as is geology (at my school they jokingly called the class “rocks for jocks”). Most schools also have classes for teachers, which might be more interesting if you are planning to be a teacher.
Talk to friends and your advisor, and perhaps sit in on a session or two of one of the classes. Your school might also have a syllabus online for the classes. Check your registrar or school web site.