很高兴为你解答.
保证正确率~!
正确答案是：C
【解析】：
It was essential that the application forms 【 (should ) be sent】back before the dealine.
申请表在截止日期前被送回是必要的.
It is essential that sb should do 做...是基本的、必要的,should 可以省略.
这里是虚拟语气,省略了 should.
类似的形容词还有：strange ,necessary ,natural 等等.
祝开心~!