Correct Answer: advisable

Solution : The correct option is the first option.

Explanation:
Advisable means something that is recommended or considered wise to do. In the context of investing earnings for the future, it is generally recommended or advisable. So, "advisable" is the most suitable antonym, contrasting with "undesirable" in the context of investing for the future.

The meanings of the other options are as follows:

• Foolish means lacking good sense or judgement.
• Unadvisable is similar to "undesirable" and means not recommended or not wise to do in a particular situation.
• Meek means being submissive or passive.

Therefore, the correct answer is advisable.