Okay friends, let's think this through step-by-step:

The sentence is talking about registering guests at a hotel, The word before the blank tells us it will be a job duty.

Hotel jobs we know of include waiter, watchman, cashier, receptionist, cleaner

• A waiter (A) serves food and drinks
• A watchman (B) guards the property
• A cashier (C) handles payments
• A receptionist (D) is the best fit to "register guests" as they greet and check people in/out
• A cleaner (E) tidies rooms but doesn't register guests

So to best complete the job duty described, the word that logically fits is choice D - "receptionist".