**Answer:**

To solve this problem, we need to determine how many times the cells divide within the given time frame and calculate their cumulative growth.

Let's break down the problem into smaller steps:

**Step 1: Calculate the number of cell divisions within 20 minutes**

Since the cell division time is 1 minute, there will be 20 cell divisions within 20 minutes.

**Step 2: Calculate the fraction of the tube filled after each division**

Since the culture tube is 1/8th filled with cells after 20 minutes, we can calculate the fraction of the tube filled after each division.

After the first division, the tube will be filled with 1/2 of the previous volume (as each cell divides into two). Therefore, the fraction of the tube filled after the first division is 1/2.

Similarly, after the second division, the tube will be filled with 1/2 * 1/2 = 1/4 of the previous volume. So, the fraction of the tube filled after the second division is 1/4.

We can continue this calculation for all 20 divisions:

1st division: 1/2
2nd division: 1/4
3rd division: 1/8
4th division: 1/16
...
20th division: (1/2)^20 = 1/1048576

**Step 3: Calculate the time required to fully fill the tube**

To determine when the tube will be fully filled, we need to find the time when the fraction of the tube filled is 1 (or 100%).

Let's calculate the fraction of the tube filled after each minute:

1st minute: 1/2
2nd minute: 1/2 * 1/2 = 1/4
3rd minute: 1/2 * 1/2 * 1/2 = 1/8
...
n-th minute: (1/2)^n

We can see that the fraction of the tube filled becomes smaller and smaller with each minute. It will never reach 1 (or 100%) because the fraction becomes infinitely small.

Therefore, the tube will never be fully filled.

Hence, the answer is (c) 60 minutes.

Bro answer is 23 min
cause generation time is of 1min so in 1 min it will divide into twice its own
so,
20min-->1/8
21min--->2×1/8=1/4
22min--->2×1/4=1/2
23min--->2×1/2=1

in 23 min whole culture tube will be filled