• The phrase "to abroad" is grammatically incorrect because "abroad" is an adverb, not a noun.
• Adverbs do not take prepositions such as "to" before them.
• Therefore, "go abroad" is the correct expression.
• Hence, Option (a) "are planning to go abroad" is the correct answer.

Additional Information

• Option (a) "are planning to go abroad" — Correct. "Go abroad" is the standard and grammatically correct phrase.
• Option (b) "plan going to abroad" — Incorrect. The verb "plan" should be followed by an infinitive (to go) rather than going, and "to abroad" is incorrect.
• Option (c) "are planning to go to the abroad" — Incorrect. "Abroad" does not take the article "the."
• Option (d) "are planning to go for abroad" — Incorrect. The preposition "for" cannot be used before "abroad."