Final answer:
The correct disadvantages of using a physical standard meter bar as a standard of measurement are that they are prone to wear and tear, can be expensive to replace, and are subject to temperature variations.
Step-by-step explanation:
The question is asking us to list three disadvantages of using a manufactured object like a platinum-iridium standard meter bar to define the meter. The correct disadvantages from the options provided would be (a) Prone to wear and tear, expensive to replace, subject to temperature variations. This is because the physical meter bars can degrade over time, making them costly to replicate and maintain. These bars also expand or contract slightly with temperature changes, which affects their accuracy as a standard of measurement. Furthermore, since the bar is a tangible object, it can only be in one place at a time, limiting its ease of access and distribution for calibration purposes.