Race, abortion, and the Israeli-Palestinian conflict are among the most uncomfortable topics for college students to discuss, according to a report on a survey of nearly 20,000 full-time undergraduate students at 55 four-year colleges and universities. The report, “2020 College Free Speech Rankings: What’s the Climate for Free Speech on America’s College Campuses?,” released on Tuesday, says that about six out of 10 students said they had censored themselves on these and other thorny issues out of fear of how others would react.