Any application that sends data to the user will face the potential that the user could manipulate or alter the data, whether it resides in cookies, GET or POST commands, or headers, or manipulates client-side validations. If the user receives data from the application, it is crucial that the application validate and verify any data that is received back from the user.

Explanation
The cloud provider's resellers are a marketing and sales mechanism, not an operational dependency that could affect the security of a cloud customer.

Application virtualization is a software implementation that allows applications and programs to run in an isolated environment rather than directly interacting with the operating system.
Sandboxing refers to segregating information or processes for security or testing purposes, but it's not directly related to isolation from the underlying operating system. Abstracting sounds similar to the correct term but is not pertinent to the question, and hosting is provided as an erroneous answer.