Bar charts are a popular tool used in project management to visually represent data and information. While bar charts have several strengths such as simplicity and ease of understanding, they also have a few weaknesses. However, none of the weaknesses listed in the options provided are accurate. Let's discuss each option and explain why it is not a weakness of bar charts.

a) Suitable only for small jobs:
Bar charts, also known as Gantt charts, are suitable for projects of varying sizes, whether they are small or large. In fact, bar charts are commonly used for complex projects with multiple tasks and dependencies. They provide a clear and concise overview of the project timeline, enabling project managers to monitor progress and make informed decisions, regardless of the project's size.

b) Cost control cannot be achieved:
While bar charts primarily focus on depicting the project schedule and timeline, they can also be used to incorporate cost control. By assigning cost estimates to each task or bar, project managers can track and compare the planned and actual costs throughout the project's duration. This allows for effective cost management and control, ensuring that the project stays within budget.

c) Optimum use of men and machines cannot be done:
Bar charts can certainly help in optimizing the use of resources, including men and machines. By clearly representing the duration and dependencies of each task, project managers can identify potential bottlenecks and allocate resources accordingly. They can ensure that the right people and equipment are available at the right time, maximizing productivity and efficiency.

d) None of these:
This is the correct answer. None of the options listed accurately represent a weakness of bar charts. Bar charts are a versatile and effective tool in project management, suitable for projects of any size, capable of incorporating cost control measures, and facilitating optimal resource allocation.

In conclusion, bar charts are a valuable tool in project management, providing a visual representation of project schedules, facilitating cost control, and optimizing resource allocation. They are not limited to small projects and can be effectively utilized in projects of any size.