To assure the unions do not make a counteroffer, the board has provided workers with outstanding benefits.
To assure that the unions do not make a counteroffer, the board has provided workers with outstanding benefits.
To assure the unions do not make a counteroffer, the board has provided workers with outstanding benefits.
To assure that the unions do not make a counteroffer, the board has provided workers with outstanding benefits.
The verb "assure" idiomatically takes a "that"-clause. The word "that" may be omitted in colloquial English, but in the formal environment of the GMAT, the word "that" always must be included.