B imo is correct and the reasoning behind it is as follows.....
The cardiovascular system, which regulates blood distribution, and the lymphatic system, which controls lymph distribution, combine to form the circulatory system, responsible for the transport of gaseous, nutritional, and waste between cells.
A. combine to form the circulatory system, responsible for the transport of gaseous, nutritional---> wrong.
Gaseous (adj.), nutritional (adj.) and waste (noun) are not parallel.
B. combine to form the circulatory system, a system responsible for the transport of gases, nutrients---->Correct
C. combines to form the circulatory system, responsible for the transport of gases, nutrients---->Wrong because Subject and Verb do not agree.
The cardiovascular.... and
lymphatic system.... is a plural subject , hence plural verb
combine must follow.
D. combine to form the circulatory system, responsible for the transport of gaseous, nutrients---->Wrong because of reasoning stated for option A.
E. together forms the circulatory system, responsible for the transport of gases, nutrients----> Wrong. Use of singular verb
forms does not agree with plural subject
The cardiovascular.... and
lymphatic system....