Visitors are welcome to experience the Center 14 or join a guided tour.
答案：(C) on their own
on their own（自行、獨自）與後面的 or join a guided tour（或參加導覽團）形成對比選項，表示「遊客可以自行參觀，或參加導覽團」。in this regard（在這方面）、one on one（一對一）、by and large（大體上）都不符合這種「自行 vs. 跟團」的對比語意。
四個選項都是常見片語，一個一個看中文都說得通，很容易憑哪個最眼熟就直接選它。one on one（一對一）尤其誘人，因為同一句後面出現 a guided tour（導覽），會讓你聯想到「一對一導覽」。
把整句讀完：Visitors are welcome to experience the Center ______ or join a guided tour.。關鍵在 or，它把兩件事擺成二選一，而後面那一件已經寫得清清楚楚，就是參加導覽。所以空格要填的必須是導覽的另一種選擇，也就是「不跟導覽，自己看」。四個選項裡只有 on their own 是這個意思（獨自、靠自己，等於 by themselves）。還有一個看得見的線索：只有這個選項帶著所有格 their，正好回頭指向句首的 Visitors，說明它講的是遊客自己的參觀方式。
in this regard（在這方面）是承接前文的轉承用語，接在動詞後面說明參觀方式並不成立。one on one 用在兩個人面對面的互動（a one-on-one interview），但一對一導覽仍然是導覽，與 or 後面那一半重複，二選一就不成立了。by and large（大體而言）是評論整體情況的說法，不是在講一種參觀方式。
on one's own（獨自、靠自己）、by oneself（獨自）、be welcome to V（歡迎做某事）、join a guided tour（參加導覽）、in this regard（在這方面）、by and large（大體而言）。
The Nobel Peace Center is located in an old train station building close to the Oslo City Hall and overlooking the harbor. It was officially opened on June 11, 2005 as part of the celebrations to 11 Norway's centenary as an independent country.
It is a center where you can experience and learn about the various Nobel Peace Prize Laureates and their activities 12 the remarkable history of Alfred Nobel, the founder of the Nobel Prize. In addition, it serves as a meeting place where exhibits, discussions, and reflections 13 to war, peace, and conflict resolution are in focus.
The Center combines exhibits and films with digital communication and interactive installations and has already received attention for its use of state-of-the-art technology. Visitors are welcome to experience the Center 14 or join a guided tour. Since its opening, the Nobel Peace Center has been educating, inspiring and entertaining its visitors 15 exhibitions, activities, lectures, and cultural events. The Center is financed by private and public institutions.