She thinks that to become a marketing manager is the opportunity she seeks for.
As @John Clifford points out, the last for is wrong. Seek doesn't need a preposition.
There are four clauses (one main and three subordinate), viz. (deleted words parenthesized):
- She thinks [that to become a marketing manager is the opportunity she seeks].
- Main clause, with a that clause complement as the direct object of thinks.
- (that) [(for
Indef/her) to become a marketing manager] is the opportunity she seeks
- That clause, with an infinitive clause complement as the subject of is, and
a predicate noun phrase containing a relative clause modifying opportunity.
Indef/her) to become a marketing manager
- Infinitive clause, with a deleted subject that's either indefinite or identical with she.
- (that/which) she seeks
- Restrictive (or "integrated") relative clause modifying the predicate NP the opportunity.
Respectively, the three subordinate clauses include a tensed object that complement, an untensed subject infinitive complement, and a tensed relative clause. The two complement clauses act as nouns (one subject, one object), and the relative clause acts as an adjective.
These are all grammatically correct. The sentence is, however, awkward, because English dislikes having long complex clauses (especially infinitive clauses) as subjects. There are a lot of syntactic rules (like Extraposition, for instance) that make sure heavy subject clauses (especially infinitive clauses) occur at the end of the sentence, where they're easier to parse.
- To be a manager is my ambition.
==> Extraposition ==>
- It is my ambition to be a manager.