The strongest evidence is Charlotte's remarks and choices. She explains that if she has any hope of having a life of some independence from her father and mother, then she must accept whatever reasonable marriage offer she receives. She prefers to marry a decent man with social status, even without love for him, than to remain dependent in her parents' home. This indicates that Charlotte--rationally aware of the realities of English society--recognized two choices for herself: (1) marriage and (2) dependence on her father. Her posititon as a gentleman's daughter didn't even allow for the option of working as a governess, a choice that was open to Jane Fairfax, an orphan raised by her impoverished aunt, in Austen's later Emma.
Another strong element of evidence for limited choices is Mrs Bennet's numerous speeches about getting her daughters married. She makes it clear that, while she seeks prestige from her daughters' marriages, she is distraught by the real possibility that parental mismanagement of funds
When first Mr. Bennet had married, economy was held to be perfectly useless, for, of course, they were to have a son.
may make it impossible for her five daughters to establish homes and independence of their own. Think of Mrs. Bennet as having Charlotte Lucas' thoughts but looking backward at the importance of independence while Charlotte is looking forward at the importance of independence. This indicates that, like Charlotte, Mrs. Bennet recognizes only two options for the daughters of a gentleman: (1) marriage and (2) independence on their father.
Another strong thread of evidence is the expectations surrounding Miss Darcy and Miss de Bourgh. They are reputed to be and actually are accomplished young women who have had excellent educations (though not at Oxford or Cambridge) and have developed artistic skill (though not at a Chicago Art Institute). They are, or will be, independently wealthy and highly placed in the upper class. With all this advantage, they have two acknowledged choices as well: (1) marriage and (2) dependence on their families.
In contrast, on the other end of the social scale (an area Austen stays away from discussing except in her later works Mansfield Park and Emma), women of the lower classes have other choices open to them: they spin wool, run dairies, work as cooks or house-cleaners or nannies, run schools, even run boarding houses (as Wickham's accomplice does). There is one unspoken choice available to upper class women, like the daughters of a gentleman (e.g., Elizabeth, Jane, Charlotte), that is alluded to during Mr. Collins' first visit to Longourn: this choice was to be a novel writer.
Mr. Collins readily assented, and a book was produced; but, on beholding it (for everything announced it to be from a circulating library), he started back, and begging pardon, protested that he never read novels.
To confirm this analysis of evidence for choices in society for upper class women, consider the scandal created when a woman, like young Lydia, goes off the expected behavioral path exhibiting behaviors from indecorous flirtations with soldiers to elopement or even not accepting a good offer of marriage regardless of love (Elizabeth was criticized per social norms for rejecting Collins while Charlotte was praised for accepting). While Lady de Bourgh's talk of education and managing her estate may seem to contradict the narrow range of two choices for upper class women, it must be remembered that education did not lead to employment for women as it did for men and that estates came either from the death of fathers or husbands--if--the estates were left to the women and, as the Bennet entail shows, many were not.