I don't think it advisable that Tom be assigned to the job since he has no experience.
advisable 后面要用虚拟语气 原句应该是...Tom should be assigned to the job...
但是should 可以省略，也就是A选项
至于其中涉及的语法，主要就是虚拟语气的一些标志词
动词的话你知道的，如suggest advise insist等等不赘述了
说一下形容词，一种是上述动词的形容词，有“可取的，适合的”意思，如advisable就是advise的形容词，又如insistent等等，还有一种是特殊的，表语气的形容词，有“重要的，必要的”如important necessary 或者“惊讶，奇怪”如strange 或者“自然的”如natural
记住以上特例，如果虚拟语气完全不明白，建议还是买本语法书，仔细地学习
这句话的意思是：我不认为tom应该被分配去做这项工作，因为他没有经验
像这类词语，advice 啊，。suggest之类的名词。或者动词，引导宾从要用虚拟语气，should do 形式should可省略
参考
一想要(desire一宁愿(prefer)二命令(order. command)三建议(advise. suggest. propose)四要求(demand. require. request. ask.insist)中，无论主句谓语动词为何种时态，从句的谓语动词都用："should + do"。should可以省略。eg： He suggested that we (should) takethe teacher's advice. He insisted that we (should) takethe teacher's advice. He demand that we (should) takethe teacher's advice. He ordered that we (should) takethe teacher's advice.
真正的宾语是（that Tom be assigned to the job since he has no experience.）
原来的语序应该是，I don't think （that Tom be assigned to the job since he has no experience ）advisable .但是由于宾语是个长句，显得累赘，所以用一个it 来替换它，作形式宾语，就成了
I don't think it advisable that Tom be assigned to the job since he has no experience.
类似的用法还有 形式主语，我举个列子，你就会明白为什么老师然你们记住很多由it 引导的句型啊，短语了，
it is no use to do or doing sth. It's +形容词+to do sth 或者that 从句
it is no use to advise her to come back to your arm. (to advise her to come back to your arm is no use)
it's healthy that wash hands before meals.( that wash hands before meals is healthy )
希望对你有帮助，谢谢！
think 后不是从句。而是think sth adj 。think的一种语法。认为什么怎么样。后可加形容词，名词。
形式宾语
