Elsa la lleva con el doctor. (The "la" represents "her", as in the daughter.)
Roberto y Marla lo compran. (The "lo" represents "it", as in the new car.)
Uds. la preparan. (The "la" represents "it", the dinner.)
Yo lo compro. ("Lo" represents, "it" referring to the gift.)
Vosotros la escribís a la abuela. ("La" represents "it", referring to the letter.)