The opposite of 'desirable' is:

ক. nondesirable

- "Nondesirable" is not a common word in English. The prefix "non-" is typically used to indicate negation, but "nondesirable" is not standard terminology.

খ. undesirable

- This option is the correct opposite of "desirable." "Undesirable" means not desired or not wanted. It is formed by adding the prefix "un-" to "desirable," which indicates negation or reversal.

গ. indesirable

- "Indesirable" is not a standard word in English. The prefix "in-" typically indicates negation, but "indesirable" is not commonly used.

ঘ. disdesirable

- "Disdesirable" is not a standard word in English. The prefix "dis-" is used to indicate reversal or removal, but "disdesirable" is not a recognized term.

In summary, option খ ("undesirable") is the correct opposite of "desirable" as it is a standard term that means not desired or not wanted. It is formed by adding the prefix "un-" to "desirable," following common rules of English word formation.