If somebody has worked for "donkey's years", it means he has worked:

• for donkeys
• for a few years
• for a long time
• for a short time
Correct Option: C  [ for a long time ]

Remark: "donkey's years": for a very long time

Though he has a lot of money, yet all his plans are built upon sand.

• based on inexperience
• resting on immature ideas
• resting on cheap material
• established on insecure foundations
Correct Option: B  [ resting on immature ideas ]

Remark:

I did not know that he was pulling my leg all the time.

• befooling me
• degrading me
• defaming me
• complimenting me
Correct Option: A  [ befooling me ]

Remark:

"Cart off" means:

• discard
• disregard
• dejected
• None of these.
Correct Option: A  [ discard ]

Remark: "cart off" means to take someone or something away, often forcefully or abruptly.