Parallelism is not the key differentiator between (c) and (d).
--
there are two problems i see in (d).
--
problem #1

a substance to promote... is unidiomatic.
the only context in which i can remember "a NOUN to VERB" is a context in which the NOUN is abstract in nature. for instance:
a way to produce goods
a reason to try harder
etc.
note that "substance" is a concrete item; it's not an abstraction like "way" or "reason".

if you're talking about concrete objects, you should replace the infinitive with something else. for instance:
* a substance to promote X (incorrect) --> a substance that promotes X (correct)
* a tool to install the shelves (incorrect) --> a tool with which to install the shelves (correct)
--
problem #2

"which is" is unnecessary and ugly.

if "which is" is followed by a description of something, you should just omit it, producing an appositive modifier:
X, which is a substance... (ugly) --> X, a substance... (better)
Person X, who is the coach of Team Y (ugly) --> Person X, the coach of Team Y (better)

if you're talking about concrete objects, you should replace the infinitive with something else. for instance:
* a substance to promote X (incorrect) --> a substance that promotes X (correct)
* a tool to install the shelves (incorrect) --> a tool with which to install the shelves (correct)

problem #2

"which is" is unnecessary and ugly.

if "which is" is followed by a description of something, you should just omit it, producing an appositive modifier:
X, which is a substance... (ugly) --> X, a substance... (better)
Person X, who is the coach of Team Y (ugly) --> Person X, the coach of Team Y (better)

，作目的状语了，翻译成<为了...，这是一个物质>不合适

平行后
其实就是纠结CD
D、a substance to的问题xdjm们说的很清楚了， 后面的but句构成歧义（与谁平行成迷）

平行没看清楚，果然要静下心的

CD的差别在于不定式和从句，promote blood clotting应该作定语修饰substance，因为thromboxane是一种substance对于是否promote没有直接决定关系，从后面平行的which prevents clotting也可以看出来，作定语修饰prostacyclin

D: ----Problem1: 描述conrete subjects不用不定式，而是定语（从句）限定修饰；
----Problem2: "which is"is unnecessary and ugly , 同位语较优！

C較D簡潔

a substance to promote... is unidiomatic.
the only context in which i can remember "a NOUN to VERB" is a context in which the NOUN is abstract in nature. for instance:
a way to produce goods
a reason to try harder
note that "substance" is a concrete item; it's not an abstraction like "way" or "reason".

Takeaway：一般同位语优于定语从句
(C) 正确, a substance that…独立主格结构作为thromboxane的同位语对thromboxane进行解释
说明; but does跳过同位语和suffices并列共同作为a low dose of aspirin的谓语动词
(D) but does…更像是跟is a substance…并列,从而作为了thromboxane的谓语动词,改变了原句
的意思

同位语和定语从句的区别？

注意 同位语和定语从句的区别（同位语说明、定语从句限制）
具体的名词不能用to修饰

从句意出发，
thromboxane，是一种物质that可以阻止血凝块，
thromboxane，是一种可以阻止血凝块的物质
第二种更加符合逻辑

but不是和同位语从句平行

这个but前的“，”决定了but后的内容并不是和substance的定语从句平行的

同位语从句是用于说明所修饰名词的具体内容的，它与被修饰词语通常可以划等号；而定语从句是限制所修饰名词的，它的作用是将所修饰的名词与其他类似的东西区别开来：
一点点阿斯匹林就足够阻止T产生，which is a substance that promotes blood clotting，（T物质的）一个特征是防止血凝块； a substance that promotes blood clotting，（T是）一种防止血凝块的物质
这里是对T进行解释，它是一种防止血凝块的物质，而不是描述它的一个特征是防止血凝块

当发现是解释描述性的语句时就应该考虑是同位语如 jonh，a student, is ........

Suffice和does平行 同时作为谓语。 描述性的从句不用which is引导

独立主格是解释说明先行词的内容，T是一个什么什么样的物质
定语从句是解释先行词的一个特征，which引导非限制性定语，是一个什么什么样的物质的T，独立主格更优