It is not letting me drop a foreign key - mysql

Question

I am trying to remove a foreign key from a table but I am getting this error below in mysql:
1025 - Error on rename of './mobile_app/Question' to './mobile_app/#sql2-4517-15515' (errno: 152)
What does this error mean and how can it be fixed?
Below is the code where I am trying to drop the foreign key:
alter table Question drop foreign key FK_Option_Table;
Below is the Question Table details (no rows in this table):
CREATE TABLE `Question` (
`SessionId` varchar(10) NOT NULL DEFAULT '',
`QuestionId` int(5) NOT NULL,
`QuestionContent` varchar(5000) NOT NULL,
`NoofAnswers` int(2) NOT NULL,
`AnswerId` int(10) NOT NULL AUTO_INCREMENT,
`ReplyId` varchar(2) NOT NULL,
`QuestionMarks` int(4) NOT NULL,
`OptionId` varchar(3) NOT NULL,
PRIMARY KEY (`SessionId`,`QuestionId`),
KEY `FK_Option_Table` (`OptionId`),
KEY `FK_IndividualQuestion` (`QuestionId`),
KEY `FK_Reply` (`ReplyId`),
KEY `FK_AnswerId` (`AnswerId`)
) ENGINE=InnoDB AUTO_INCREMENT=76 DEFAULT CHARSET=utf8
Below is the Option_Table details:
CREATE TABLE `Option_Table` (
`OptionId` varchar(3) NOT NULL,
`OptionType` varchar(20) NOT NULL,
PRIMARY KEY (`OptionId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
There are 24 rows in this table and below is a sample:
OptionId OptionType
O1 A-C
O2 A-D
O3 A-E
O4 A-F
...

Answer 1

Those are index's not foreign keys you have. I see no foreign keys in your create statements.
Example to create foreign key.
ALTER TABLE `Question`
ADD CONSTRAINT `SessionId`
FOREIGN KEY (`SessionId` )
REFERENCES `Option_Table` (`OptionId` )
ON DELETE NO ACTION
ON UPDATE CASCADE
, ADD INDEX `test_idx` (`SessionId` ASC) ;
ALTER TABLE `Question` DROP FOREIGN KEY `SessionId` ;