The answer to this is very direct and can be given in three or four words. Before we plunge into the task of revealing those three or four words, let's set up some of the background to focus the reason behind who went to visit Elizabeth while she was away from the Parsonage (i.e., Hunsford: "must make Hunsford extremely dull to a young lady like yourself") reading and rereading Darcy's letter of explanation.
You may recall that Colonel Fitzwilliam met Elizabeth while on one of her solitary walk through Rosings Park, and they turned and walked back to the parsonage together. During this walk, Fitzwilliam all but expressly stated his affection and fondness for her. At the same moment, though, he pointed out that as a younger son, he sought a woman of wealth to marry. Elizabeth was surprised and surmised, correctly no doubt, that Fitzwilliam had intended her to understand his position and his inability to offer her a proposal even though he did prefer her.
"I speak feelingly. A younger son, you know, must be inured to ... dependence. ... in matters of greater weight, I may suffer from want of money. Younger sons cannot marry where they like. ... there are not many in my rank of life who can afford to marry without some attention to money."
"Is this," thought Elizabeth, "meant for me?" and she coloured at the idea; ... (Chapter 33; Vol. II, Ch. 10)
You will remember also that on the night preceding the day on which Elizabeth received the letter from Darcy, he himself had proposed marriage to her. You will recall too that she most decidedly rejected his offer. She went further than that to accuse him of damaging Wickham's life and of ruining Jane's happiness. In fact, it was the things Elizabeth said that precipitated the letter he wrote and gave to her.
[Darcy in letter]: "I write without any intention of paining you, or humbling myself, ... my character required it to be written and read. You must, therefore, pardon the freedom with which I demand your attention; ..." (Chapter 35; Vol. II, Ch. XII)
Also recall that Colonel Fitzwilliam and Darcy unexpectedly hastened their plans to leave on the upcoming Saturday: "The two gentlemen left Rosings the next morning." Now, remembering that two men were in love with Elizabeth (to one degree or another), and remembering that they wanted "to take leave" of her before hastening unexpectedly away, it will come as no surprise to learn the names of the visitors who sought Elizabeth at Hunsford while she was away walking and reading. The two who came to call on her were, first, Darcy, staying "only for a few minutes," and, second, Colonel Fitzwilliam, who had remained "sitting with them at least an hour, hoping for her return." In four words, the answer is Fitzwilliam Darcy and Colonel Fitzwilliam.
She was immediately told that the two gentlemen from Rosings had each called during her absence; Mr. Darcy, only for a few minutes, to take leave—but that Colonel Fitzwilliam had been sitting with them at least an hour, hoping for her return, and almost resolving to walk after her till she could be found. (Chapter 36; Vol. II, Ch. XIII)