the matter ---your carelessness is very serious;应填relate to/

elated to (最好——具体看下面解释)
relates to
relating to
都行:
具体见下面relate的用法:
这里用到的是:
不及物的第三种意义:
3.涉及；与…有关(常与to连用)：
It is a problem that relates to geology.
那是一个与地质学有关的问题.
如果没有S,就用relating to 也行.变定语从句,为现在分词定语.
译为:与你的粗心大意有关的那个问题,很严重.（严重的是问题本身~）
如果用:related to 归于你粗心大意的毛病,很严重.（严重的既是问题也含带有你本人的错）显然这个意思 要好些.
relate [ri'leit]
vt.
1.讲,叙述：
He related what he had seen in the theatre.
他把在剧场中所看到的情况叙述了一遍.
2.使联系；显示…之间的关系：
The Belgian language is closely related to French.
比利时语与法语密切相关.
vi.
1.与…有某种联系(或关系)(常与to连用)：
She is concerned only about what relates to herself.
她只关心与自己有关的事.
2.相处得好；体贴,关怀；适应(常与to连用)：
The two brothers can't relate to each other.
兄弟二人互不体贴.
3.涉及；与…有关(常与to连用)：
It is a problem that relates to geology.
那是一个与地质学有关的问题.
4.适用；溯及既往：
These regulations do not relate to children under the age of twelve.
这些规则不适用于十二岁以下儿童.
5.符合；互相一致：
What he said did not relate with the facts.
他说的话与事实不符.
6.发生共鸣；认同；欣赏；理
They can't relate to modern literature.
他们对现代文学很不理解.