The Problem:
In a class of 55 students, the number of students studying different subjects are as follows:
- 23 students study Mathematics
- 24 students study Physics
- 19 students study Chemistry
- 12 students study Mathematics and Physics
- 9 students study Mathematics and Chemistry
- 7 students study Physics and Chemistry
- 4 students study all three subjects

We need to find the number of students who have taken exactly one subject.

Step 1: Visualizing the Information
To solve this problem, it is helpful to visualize the information using a Venn diagram. We can draw three intersecting circles to represent Mathematics, Physics, and Chemistry. We can then fill in the given information in the diagram.

Let's label the circles as M, P, and C for Mathematics, Physics, and Chemistry respectively. We will also label the regions where the circles overlap with two subjects, such as MP for Mathematics and Physics.

Step 2: Filling in the Information
Using the given information, we can fill in the Venn diagram as follows:

- The number of students studying Mathematics (M) is 23.
- The number of students studying Physics (P) is 24.
- The number of students studying Chemistry (C) is 19.
- The number of students studying Mathematics and Physics (MP) is 12.
- The number of students studying Mathematics and Chemistry (MC) is 9.
- The number of students studying Physics and Chemistry (PC) is 7.
- The number of students studying all three subjects (MPC) is 4.

Step 3: Calculating the Number of Students
To find the number of students who have taken exactly one subject, we need to subtract the number of students in the overlapping regions from the total number of students in each subject.

- Number of students studying only Mathematics (M - MP - MC - MPC): 23 - 12 - 9 - 4 = 23 - 25 = -2
- Number of students studying only Physics (P - MP - PC - MPC): 24 - 12 - 7 - 4 = 24 - 23 = 1
- Number of students studying only Chemistry (C - MC - PC - MPC): 19 - 9 - 7 - 4 = 19 - 20 = -1

Since the number of students cannot be negative, we can conclude that there are 0 students studying only Mathematics and only Chemistry.

Therefore, the number of students who have taken exactly one subject is the sum of the number of students studying only Mathematics, only Physics, and only Chemistry.

Number of students = |-2| + 1 + |-1| = 2 + 1 + 1 = 4

Thus, the correct answer is (d) all of these, as there are 4 students who have taken exactly one subject.

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