Option D: 39, 29, and 11 respectively
To solve this problem, we need to determine the number of students who can speak Hindi, the number who can speak only Hindi, and the number who can speak only English. We are given the following information:
         ○ Total number of students = 50
         ○ Students who can speak both English and Hindi = 10
         ○ Students who can speak English = 21
     Let's denote:
         ○ $E$ as the number of students who can speak English.
         ○ $H$ as the number of students who can speak Hindi.
         ○ $B$ as the number of students who can speak both English and Hindi.
     From the problem, we have:
         ○ $E = 21$
         ○ $B = 10$
     Step 1: Calculate the number of students who can speak only English.
     Students who can speak only English = $E - B = 21 - 10 = 11$.
     Step 2: Use the total number of students to find the number of students who can speak Hindi.
     The total number of students is the sum of students who can speak only English, only Hindi, and both languages. Therefore:
     $$\text{Total students} = (\text{Only English}) + (\text{Only Hindi}) + (\text{Both})$$
     Let $x$ be the number of students who can speak only Hindi. Then:
     $$50 = 11 + x + 10$$
     Solving for $x$:
     $$50 = 21 + x \implies x = 50 - 21 = 29$$
     Step 3: Calculate the total number of students who can speak Hindi.
     Students who can speak Hindi = Students who can speak only Hindi + Students who can speak both English and Hindi = $x + B = 29 + 10 = 39$.
     Therefore, the number of students who can speak Hindi is 39, the number who can speak only Hindi is 29, and the number who can speak only English is 11.
     Thus, the correct answer is Option D: 39, 29, and 11 respectively.