The infinitive form of a verb looks like this:
the infinitive marker TO + A VERB
Having this in mind, the infinitive form of a verb from this sentence is C. too seek (as well to strive, to find, not to yield).
A is simple present tense, which means that it is not infinitive. B is not even a verb, it is an adverb. D is future simple tense, which means it is not infinitive.